According to the results of cross-sectional studies, on average older people learn new things more slowly than younger people because

I believe the answers are:
Older people lack practice (they tend to reduce the usage of their brain when opting to do more relaxing activities)

Older and younger people have different learning styles (younger people much more likely to be involved in trial and error learning)

Older people may be less motivated to learn new things than younger people
(younger people tend to be more ambitious and older people tend to want to settle down and enjoy their life)

Reaction time is slower in older people compared to younger people (as people grow older, our cognitive ability tend to deteriorate, which resulted in slower reaction time)