What do you mean by make that decision? Just that they can't get to the conclusion, but the "symptoms" (for lack of better term) are there? That they may act (or want to act) like the opposite sex, but can't (shouldn't?) make the decision to do so?
Damned if I know, I'm certainly no expert, but as you work with children (?) I thought you might have a better opinion. Seems to me that it would be fairly inherent and immutable, that you either are, or aren't, the thing that you are. If a person believes they are female, or at least stereotypically female and want feminine things, it would be something they've likely always wanted.
I wanted GI Joes and to be outside, never wanted clothes nor cared about my appearance, that makes me stereotypically male. None of that ever really changed. Even within stereotypically male attributes there are differences, and anecdotally I've never seen those really change over time.
I do imagine it could be mental, and changes in that could alter that state of mind, such as a stroke and similar things. Other than "deciding" that they are, in fact, something "trans", I don't think it much matters what age they are.