> Young children seem to pick it up with ease. It cannot be that hard

It is other way around. Children can pick up a lot of skills that adults struggle at, like languages for example.

Plenty of research has shown reduced plasticity of the brain has stronger correlation to learning speed and ability as it grows old. Most breakthrough research is usually at age 40 or less or chess grand-masters fade in skill after. 25-40 is probably age group where the optimal balance between knowledge experience and learning ability for best outcomes.