I disagree with maybe pilot about the immaturity of a 20 yr old. That is absolute nonsense,i myself have seen many older first officers with the sense of a child, It has nothing to do with age its personality and ability that would be the issue when you take away your so called "variables"
ALLOW,
one can be immature at any age but a 20 years old/200hrs pilot is obviously more likely to be one than someone who has the same amount of hours but has seen a little more of the world (traveled,done other jobs,got a mortgage,bought a car,paid his bills,had a couple of girlfriends) or someone who has the same age but a few more hours flown in other airlines, dealing with ops/pax/wx/FTL's/T&C's and so on.
I fly for an airline with a cadet scheme and sometimes find myself having to act more like a father than a Captain.....