Guys tend to mature at a slower rate than girls and therefore a 20 year old guy will still be a baby. It is possible also that part of the reason for this is society and the way things are set up where a guy doesn’t feel the need to be grown up until he is in shidduchim age. If the trend changed and guys started getting married at 20 then I think the pressure would be on to grow up faster.
That being said, I think many girls at 19 are very immature even though they are dating. There’s often a big difference between a 19 year old and a 21-22 year old girl.
It used to be that girls are mature by 18 or so but that no longer applies. Just look around. People in general have no interest in growing up. We aren’t as mature as our parents were at our respective ages. (I refer to anyone under 35) that’s just the way things are today.
Maturity though doesn’t necessarily have to correlate to age. Its usually a combination of age and what the persons temperament and life has been like.