You'd think what you learn when older you would remember better, not forget. Things you learn when very young was so long ago why would you remember them?
Children tend to learn a lot of fairly simple information very rapidly, and in early life we learn a good deal of what we need just to live our daily lives so retain by that regular use.
Later we learn more complex topics, or the same academic subjects but to greater depth, and these include many we later forget by disuse.
As young adults we add to the daily-life skills, as in learning to drive, to look after our homes, or manage our finances, and maintain those skills by frequent use (assuming no neglect). We might also learn a foreign language not taught at school, to enhance travelling abroad; or take up a hobby that involve a good deal of learning and practice to be able to enjoy properly, such as playing a musical instrument.
With age, it becomes harder to learn anyway, generally, though some people are able to study particular subjects to high levels even in old age. I've a friend who gained a PhD in a scientific field, as a retirement hobby.
