While everyone else has correctly stated that it largely depends on the two individuals... just to try to give you a straight answer, most of the marriages I have seen amongst my friends, the guy gets married at the age of 28-32 and the wife is generally 3-5 years younger than the husband. Seems about right as to what majority of the folks do. Exceptions are there, but age difference is no guarantee that the marriage will be a success.
i dont think age matters, its the way of thinking on the same level does...
age ain't nothing but a number like aaliyah said ] :)
all this talk about mental judgement etc sounds good but then again a little bookish. If you look around I'm sure you'll find that in almost all of the marriages among your reletives guys are 3-6 years older. One of my uncle is 10 years older than his wife. But then another aunt of mine is 4 years older than her husband.
In reality I'd say men prefer younger women and women older men. That's how the nature is. Of course I agree there are circumstance which are other wise but comparing to the usuall run of the mil (guys are older) that ratio is too low.
PA : 15 to 20 
id say maybe 5 to 6
main thing is mental adjustment,
for a succesful life... both should have same mental level or atleast should have great understandings, some how it is related to age as well,
too much age diffrence, like 15-20 is not acceptable, but 2-10 is still ok,
but depends how old girl is!
if she is just 14 and guy is 20, it may have so many conflicts, but if the girl is 25+ and guy is even 40+, they can make a good couple, but all depend on mutual understanding!
For a successful life along with love there should be a understanding among each other :)
The example of our Prophet sallalahu aleyhe wasallam shows that age difference is not that big a deal. Some people mature very early and some dont mature until their 40s so as long as there is understanding between the two i fail to see how age matters.
Seeing that women live on average 3-5 years longer than men in most culture (where they don't die in childbirth as in poorer countries) ... maybe the "natural" age difference is this - woman 3-5 years older than the man. Just a thought.
My opinion is, it will happen with whomever it is meant to be, regardless of age difference.
One of my good friends stated that since women live longer than men on average, she will surely marry a guy a few years younger than her, just as every other woman in her family had done.
As Aleezey mentioned that it's the mental adjustment that matters and not the age difference.. that a girl has to be younger then her husband.