There are many 20 year olds in the RH seat, you can hold a CPL from 18.
I think it is a question of maturity, I have met some 18-20 year olds in training who's attitude left a lot to be desired. However, I have also met a few who were outstanding,very responsible people (in my opinion, of course).
About me, I am 20, did modular training, and am starting my A320 type rating on Monday
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See you in the skys...
EpsilonVaz