We don't know, but epidemiological data hints at which one is more likely.

ADHD diagnosis requires "onset" before age 14, yet there is such a thing as acquired ADHD (due to brain damage).

And of course the genes don't change during the latter, nor did they change when the age of onset criteria was raised from 12.

And even though both autism and ADHD was described more than a 100 (and 200) years ago it took a long time for AuDHD to be noticed. (2013)

The maps we have are bad, the territory is treacherous, and even if we assume the genes are unchanging the environment they find themselves in does seemingly faster and faster.