Maturity isn't defined by what you do for a living, neither is it - to a certain extent- defined by age. Maturity is defined by how you react to certain situations and by your attitude in general. If for instance someone tries to point out a mistake you made an immature reaction is to say "shut up, noone asked you anything" instead of "thanks for pointing it out, I'll try to improve it" whether you mean it or not.
You never struck me as immature, but it seems you misinterpreted the word
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