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Pick a name and go with it.

If the fact of the new character being future Adrien isn't a secret from the reader, you don't have to worry about names that spoil the surprise.

Use whatever name Adrien himself will use. He's not going to refer internally to his future self by his own name or by something long. He'll pick a name pretty quickly, because his brain is going to need to wrap itself around this situation and that's an important way of doing it.

You know the character best, but here are some names you might choose from:

• Future Adrien


• Future Me


• Mr. Smith (whatever Adrien's last name is)


• (whatever Adrien's middle name is)


• Some nickname neither Adrien will find insulting, like Adie or Enn.


• A childhood nickname that isn't based on his name.

Establish this name very quickly and then use it consistently.

If it were me, I would pick a name with a common, well-known nickname, and then call the younger version exclusively by the nickname, and the older version by the full name --for instance, "Andy" and "Andrew."

Andy was starting to think he didn't like "Andrew" very much. Apparently, as he got older, he was going to turn into an even meaner version of his dad. "Not if I can help it," he muttered to himself.

Meanwhile all Andrew could think about was what an inconsiderate brat he used to be.

This makes clear the connection between the two of them, without making it possible to confuse them, as long as you make sure no one ever calls one of them by the wrong version of the name.

(I believe I've seen this done successfully somewhere, although I can't remember where...)

The Magic 2.0 series by Scott Meyer has this situation with a core character (so it's not a passing situation). The narrator and the characters identify the two as Brit the Elder and Brit the Younger. When more time-travel shenanigans happen, we also encounter Brit the Even Elder and Brit the Much Younger, which doesn't seem sustainable but these are shorter scenes.

An alternative to these kinds of sequential references is "place of origin" references, if that's meaningful to you. If you have a time-traveler from way in the future visiting a modern-day character, they might be New York Bob and Alpha Centauri Bob or, more ominously, Last-Days Bob and Post-Cataclysm Bob. Any reference that readers and all involved characters will (come to) understand and that is durable works.

Clarify each character's motivation in the scene.

The best way to keep dialog straight is when the dialog makes sense. This shouldn't be any different than any other two characters engaged in dialog. The limited-POV MC certainly won't be confused by which one of the two is speaking. Stick closely with him since he can lampshade the weirdness of talking to another version of himself, even to the point of copying your question:

"What am I supposed to call you, Other Me…? Are you Older me, or
Earlier Me…?"

"Shut up and listen!"

Presumably "other Adrien" has done this before and is less impressed by the moment, but also needs to communicate something of importance and wants his younger self to focus. They should have clear but different motives in the scene, so readers should be able to tell which is speaking just from their dialog.

Use "he said" (etc) as sparingly as possible, only when absolutely needed. It's possible that too many of these "he said" indicators is what's actually leading to the confusion, as readers mentally switch between speakers assuming the indicator is telling them something different than expected.

In other words, too many signposts is possibly more confusing than letting the dialog sort itself out.

This is a frame challenge. I think your issue could also be that your characters do not have a distinct voice.

A 15 years old sounds different from a 20 years old. I am not referring to the timbre of their voices, which should also be different. The vocabulary is different, the ability to articulate their thoughts is different. Even their logic, their values, their fears, dreams and desires may be different. Regardless of how you call them, these two characters should be easily identifiable by your readers.

Suggestions:

• timbre and mannerism: the younger character has a higher pitched voice. He may squeak, chirp and squeal at times. The older character has a deeper tone. He may also speak slower.




• vocabulary: the younger one repeats himself more often. He uses simpler and more common words. He may try to use words heard from the older one, but not necessarily in the right context. The older one may use slang and jargon to sound cool.




• articulating thoughts. The younger one speaks his entire line of thinking, it is linear, and may not branch into a lot of alternative considerations. The older one frequently considers alternatives, which he expresses use longer sentences, and conditional clauses.




• logic: the kid has a greater more vivid imagination. Hemay consider things that are outright impossible. He may be more prone to believe based on empathy alone. The older one is more likely to question a statement.

... what are some good ways to distinguish between them without taking the reader too far out of the story?

1. 
   

   First introduce the main character (unless you prefer to introduce the alternate first).

   
   
   

   • Provide an overview that establishes some characteristics of the person.

   
   

   • Reveal additional information about the person later to further develop our impression of them.

   
   

   • Further details can be kept from the reader, to add mystery or to be revealed for a plot twist, etc.

   
   


2. 
   

   Introduce the other character, either side by side or isolated in a seperate scene.

   
   
   

   • Whether or not it is immediately obvious to the reader that there are two persons is dependent upon how you wish to write the plot.

   
   

   • How much information is provided during the initial reveal is again dependent upon the way the story is written.

   
   

   • Introduce the second person in a manner similar to the way one would present an identical twin.

   
   

   • Since the two characters are five years apart describe the dress and actions of each slightly exaggerated away from each other, for example the 15 year old might seem more like a 14 year old in behavior and clothing while the 20 year old would dress and undertake activity more suited to one 22-25 years old.

   
   


3. 
   

   Since each has a separate timeline (they didn't suddenly divide) each has come initially from the same place but most recently, most likely, from different places. Their recent past and whom (and how) they have interacted with people will be different.

   
   
   

   • The younger one would have most recent memories (and clearer recollection) of events that occurred when they were younger, while the older one would possess information unknown to the younger self.

   
   

   • The older one would know more people, or others persons and events that are more recent. Decide how to explain which other characters are from old or recent past and then the interaction of each persona with the person or event will hint the reader as to which one is being referred to. Exactly how clearly you convey that will determine how clearly the reader perceives the one being referred to, depending again on how you are telling the story you can adjust that; and make it more obvious later.

   
   

   • The older one would have additional abilities and possessions. They might own a car, have a job, even have had a divorce; all things that the younger self lacks.

   
   


4. 
   

   Where a future self goes back to a past time they would know of a future that might never exist, this could be a source of unwarranted concerns not understood by the characters of the time.

   
   
   

   • The older self would seem like the older brother of his time-twin to his teenage peers. Sometimes this could be a characteristic seen as useful or one of which they would be envious, other times this may be perceived as domineering or one whom knows too much (doesn't let loose, too mature). This depends upon the personalities of the younger and older selves and how the writer chooses to present them.

   
   

   • The older self might choose to set a good example for the younger persons or might have taken the wrong road and it is the younger group that sets the better example. Depending on how the story is to be written the two groups (the older self, and the younger self and peers) could interact with each other from an entirely different perspective; depending again on how much personal development is bestowed upon the elder.

   
   

Keep your younger audience in mind while writing and introduce the older person's experiences and new responsibilities so they are not foreign to the reader, while portraying the older as more knowledgeable but possessing knowledge not either deemed essential or even relevant in the younger time line. Similarly the younger self might have goals that proved impractical or were never realized by the elder, either might put it upon the other to embrace or abandon these goals.

The younger and older version would be no different from twin test tube babies born five years apart and raised by separated parents, if that's an easier way to summarize the concept.

Whether, in the end, each persona seems alike or quite different, is successful or not, well liked or despised depends on the direction of growth each experiences; and the degree to which each is capable of affecting the other.

If making the distinction between the two is "too far out of the story" then they are really like same aged identical twins whom spend a lot of time together and share common interests; in a written work constantly having to explain whom is who becomes tiresome and should be avoided. If they are not to be so different then the relevance of whom did what should be left for the screenplay and not be belabored in the book.

Use prime marks: Jim, Jim', Jim'', Jim''' etc.