I was diagnosed at age 14... many many decades ago.
It pretty much pushed me in the deep end (fired from my first job, OK it was just a summer teenage job) as far as "facing up to the real world". If it did have an effect I think it forced me to skip the irresponsible-teenager step in life, and maybe in a John Hughes movie sense that was a loss.
(BTW, as far as John Hughes movies go, the only one I ever liked was Ferris Bueller!)
I was dx at 17. Didn't think it really arrested development for me because I was in denial for so long. Did ok in school, although the symptoms I had affected my grades before I was dx'd. People might react differently, but looking back on it, it wasn't easy since at that time you are gaining autonomy while your parents are overly concerned about your health, which might feel overbearing.
I have had some confidence and alcohol issues while in college and after, which has probably held me back a bit. As long as she is taking care of herself and has a good support system in place, that will help her out a ton.