Imma be real with you, dawg: it's almost always gonna be characters who are 18 or older, and for many reasons. I personally have a lot of playable characters who are grown adults in their 30s or even 40s going out trying to save the world or whatever, and I try to include older family members because you know what, it could actually be way more interesting than having them be dead or abusive or brainwashed.
Kid heroes saving the world has always been like... not believable to me? Anybody younger than 15 being better heroes/fighters/whatever than hardened veterans is pure plot convenience nonsense, and the more serious and complex your story is, the more I expect this kinda trope to be deconstructed. It really is that kind of thing where you have to decide what mood you want to go with, and what your target audience is.
There's only two characters I can name of mine that are both 14 and playable, but neither of them are even half ready to tackle the hardship and horrors my world contains at their current age. I still include them to show off that they have a wild amount of untapped potential that will explode when they grow older, but right now they are not viable.
Plus like
@Punamaagi said, there's a lot of variance even among adult ages and that tends to invite a lot more opportunities to have interesting characters.