ahoy I also just joined hehe, but I agree with Renai!
you are ultimately the person who's interpreting your f/o(s), and if you feel like they've grown up with you, then they've grown up with you! if it feels like they just can't or shouldn't, etc, maybe it is time to let go of them and move on. but i think that with growing up with your f/o(s), it opens up a great opportunity to explore character evolution/design changes, which is something I enjoy as an artist ^^
I'm almost 19, and have also been self-shipping since middle school. I intend on letting my current fictional partner grow all old n' gnarled with me, it feels very romantic hehawhaw