Since Roxas was 16 before but now probably 17 after the one year time skip.
Roxas is 15 in KH3, assuming he is the same age as Sora. So after the one year skip he should only be 16. Everyone seems to think Sora is 16 in KH3 but he's not.
I thought Sora was 16 since Nomura said (at least some ppl say they saw it from an interview) a year should have passed in kh3.
Official KH3 Ultimania lists Sora's age at 15 at the start of KH3. There isn't any indication how long the game actually takes, and the characters certainly never seem to acknowledge a birthday of any sort so I wouldn't assume he had one during the game.
However, I think Nomura is deciding to be a little loosey-goosey with actual ages and for good reason since we have people like you said: going into the realm of darkness for certain periods of time, people putting themselves into realms of sleep voluntarily, people with artificial bodies, people being rejected from reality...it's all just a mess at this point that I'd be surprised if the characters themselves even know how old they are. That's probably why we stopped getting absolute ages for people around BBS, Nomura probably knew it was going to be an issue moving forward.
Also while we do know Sora's "birthday" that is in more of an official capacity considering we don't even know if the KH universe has the same system of "years" that we do, or if "March" is even a month for them.
Moving forward we should probably stop trying to decide on hard numbers for ages and using what their age "should" be to determine their appearance and instead just think about it from a character design perspective. Would it be better design-wise for all three of these characters to end up looking more distinct from each other? Or for symbolism's sake should they stay similar regardless of the situations?
I say start giving them unique features. Personally I just want it so that if a character jumps on screen, regardless of their outfit, I should be able to tell who it is right away. Maybe a vain hope in the KH universe of duplicates, but I'd imagine with Nomura being a character designer he should be able to find ways to subtly change their hair or other features to achieve that.
While it is cute for Roxas and Ven to be like a weird set of twins, and there are some funny jokes that could be had at that expense, I think they are very different personality wise and it would be best to relate that in appearance to the audience. Even if it is a small difference, I think Ven being a bit shorter and having lighter hair would be a nice start. I always thought Roxas should have a dirtier brownish blond color with Ven being very light blond. Then we could set the Sora trio up in a line and see a nice color gradient progression, haha. Height wise I hope Ven stays the shortest.
I hope they move in that direction visually, but then again, they did bother color correcting Roxas and Ven's hair in the ending scenes, so...