I think the border symptoms do get worse and your medication resistance might increase (also, if your weight is increasing the same dosage will be less effective)… but in terms of the primary symptoms - I think they decrease. Or rather, I think our ability to cope and mask increases as we age. I’m currently a manager and keeping on top of other people doing their tasks and having their needs met is something I never imagined being able to do but now that I’m doing it it’s all sort of just working.
We get more skilled at dealing with our executive disorder as we age. We plan better for the possibility that we’ll be in an off mood or down a rabbit hole… we are (hopefully) more comfortable with sharing our condition with our relations so that partners are aware of our difficulties and strengths and can react accordingly - especially once we can overcome the deep shame about ADHD being a personal failing that was beaten into most of us… we give our brain time to indulge and better set internal expectations and accept that some weekends we’ll never touch a feather duster while other weekends we’ll deep clean our living space and just… be okay with that.
I’m sure in the extremes it’s rough and there are studies that our ADHD is an increased risk factor for dementia so… eventually we’re all probably fucked… but hopefully that’s a long way off.