My nephew is on meds and is 16 years old now. He's been on meds since he was probably 11 or 12. It was also night and day when his meds were effective.
However, last year, probably due to hormonal changes due to puberty, his meds lost efficacy and he had extreme issues in school (failing grades, anger, etc.). His parents met with a school administrative group and they immediately figured it was probably his meds. That was when they brought him back to the therapist or psychiatrist (whichever prescribes meds) and had him determine that he needed either new meds or a higher dose of what he was already taking.
Again, his behavior difference was night and day and he's doing fine.
EDIT: His parents initially struggled for a long time with the decision of whether or not to medicate, taking all the usual steps to try to help with their son's issues. But after they started with meds they knew they had made the right decision.