Do ADHD meds only work for kids — do they even do much for adults?

That's an outdated assumption, not a real evidence gap. ADHD medication is well-studied and shown to be effective in adults too, the mechanism doesn't stop working just because someone's brain has finished developing, ADHD itself is now well understood to persist into adulthood for the majority of people diagnosed as kids, and plenty of people are diagnosed and treated for the first time as adults with real, measurable benefit. Where the 'it's really a kids' thing' idea probably comes from: ADHD research and treatment guidelines historically focused heavily on children because that's where diagnosis rates were concentrated for decades, adult ADHD as a recognized, actively-diagnosed condition is a more recent shift in how seriously it's taken clinically, even though the underlying condition was always there in adults too, just unrecognized or dismissed. Adults do sometimes respond somewhat differently than kids in terms of dosing and sensitivity, which is a real clinical consideration your provider will account for, but that's a dosing nuance, not evidence that the medication class doesn't work in adult brains. If your family member's mental model is 'kids outgrow needing it,' that's a separate (and also not fully accurate) idea from 'medication doesn't work in adults,' worth gently separating those two claims if it comes up again.

Diagnosed and medicated for the first time at 40, real, noticeable difference within the first couple weeks, definitely not 'too developed' for it to matter. Wish someone had pushed back on that assumption for me decades earlier honestly.