I was 30 and in grad school before I knew about my own ADHD. It’s going to be ok for your kiddo, and you’ve done well to help your teen get appropriate resources.
Starting stimulant drugs is a great experience for a lot of teens and adults. It vastly reduces the difficult in initiating and sustaining tasks and can reduce the mental fatigue that less-desirable tasks induce. In turn, noticing the effects of medication can, for some, ease the self-criticism that can occur when required tasks don’t get completed.
In some cases, people experience bumpiness with appetite, mood, and sleep after starting a stimulant. It’s best not to over-schedule oneself and to leave space for things like naps and snacks when possible. Eventually, you find the right dosing and timing, but it can take some weeks or months of making adjustments.