First if both people are mature, then there should be no problem. HOWever, most 20 year olds are not ready for marriage in my opinion, after 21, 22, seems better. Again, depends on each person, and why they want to marry each other, and communications extremely important.
*To Tony, Taylor, and Robyn: Good to see what u said. I'm 26 years old and possibly starting to fall for a 44 year old man. We met around a year ago and have been better and better friends ever since. He's divorced twice with 4 grown children, youngest is a few years younger than me, oldest are around my age. Is it at all weird that some his children are around my age? Honestly if he was my dads age<54> that would be a bit weird for me..