I think that the age gap issue is much more about maturity and development
than about some arbitrarily established number.
However, it is precisely for that reason
that I think a 21 year old and a 16 year old is an extremely inappropriate match. All of the teenage years and the very early 20's are highly formative years
. These years tend to contain enormous development and maturation, both physically and mentally. I would think that virtually anyone who has lived through these ages can vouch for this. This means that the difference in maturity between, say, a 19 and a 21 year old tend to be much greater
than the difference in maturity between a 31 and a 33 year old. If a 21 year old feels on the same level as a 16 year old, there is something seriously
messed up in the development of the 21 year old.
My own development over that period of time was enormously significant.
While a huge gap in development and maturity is arguably somewhat unsettling in itself, I think the greater problem is that it creates a situation wherein one of the members of the relationship can be easily taken advantage of. It seems to promote inequality and selfish manipulation. I would have trouble trusting the motives of anyone who was intent on seeking out someone much less
developed and mature than they are.