My personal experience as a CFI. 20's usually pick things up rather quickly, especially multi-tasking skills such as radios. After 30, they learn fine, but not quite as quickly. After 40 it's noticeable slower...after 55, watch out.

Exceptions abound of course. People who have military or LE careers tend to do better, and I imagine that any work or recreational experience which requires hand-eye coordination or dynamic, time-sensitive communication (stockbroker, NYC Cabbie?) would also.

Any experienced instructor will have noticed this. The 55+ YO recently retired executives are the worst...they are usually completely removed from dynamic, interactive communication (in their world they talk, and the everybody listens patiently). Everything is on their schedule, at a pace comfortable to them...it's been a long time since they have pressured or challenged by their immediate environment. Their challenges are large in scope and long term.