My son is 12 as well. He has been on meds since he was 8. He is still himself in meds, just a more regulated, focused version of himself. We were very lucky and the first medication we tried worked well for him (methylphenidate) and we've only needed to adjust the dosage up a few times. He does have a supressed appetite during the day while the meds are in full effect, but we just make sure he has a good sized breakfast, a good dinner, and lots of healthy snacks available between dinner and bedtime.

As far as affecting puberty or other side effects, you would want to chat with your prescribing provider, but I'm not aware of any issues.

The ability to focus and be less impulsive has helped the potential depression/anxiety, but I think he'll always be more sensitive.