So a couple who are both 20 is like a 22 yo woman dating an 18 yo guy...
Most Helpful Guy
Generally (and we are talking huge sweeping generalisations here) guys and girls focus on different aspects of their lives... and sometimes this will look like immateurity.
For instance a lot of grown women naiivly believe in starsigns, true love and such long after men. But on the other hand guys will do stupid, physically dangerous, things in the name of a laugh far later in life.1
Most Helpful Girl
I'm not sure about the 2year thing but generally women are more mature than an equal aged guy. That's one reason I prefer to date older guys.3