I've definitely learned a lot of math from Wikipedia. I don't generally do the proofs myself, so I don't really have any of the elusive "mathematical maturity", but I definitely have learned a lot of abstract algebra, category theory and mathematical logic just by reading the definitions of various things on Wikipedia and trying to understand them.

On the other hand, I am pretty motivated to learn these things because I actively enjoy them. Other branches of math, I am much less interested in and so I don't learn that much. But it is possible!