I think a big part of "mathematical maturity" is knowing how important it is to have someone else look over your proofs. Of course you can learn a ton on your

I think a big part of "mathematical maturity" is knowing how important it is to have someone else look over your proofs. Of course you can learn a ton on your own by reading and working exercises and doing research, but there is no substitute for collaboration. Mathematics is inherently a social activity, even if the bulk of it can (counter-intuitively) be done in relative solitude.