A friend learnt French and piano in her mid 30s and later. Then she went lazy as potato and pretty much is horrible at both. Never practised. Now she is in late 40s and rather spends time scrolling Twitter (yup!), Instagram, and Youtube or overworking. So, I seriously doubt the true brain capacity is the problem in 30s or later. She also had somewhat of a life in her 30s and then she never made an effort to keep having that life (something I am inching towards in my late 30s now; also in the geography we both are this is inherently difficult - no, it's not an excuse) and that also contributed to losing a lot of her personality, or that's how she puts it. So I think to use the brain the brain needs to be primed and well oiled et cetera.