Do you think 21 and 29 is a big age difference? What about 21 and 25? Do you feel like those differences would usually be significant, or does it depend more on the individuals?

And please no "If you love each other age is just a number" type answers, I'm really talking more about details and maturity levels here.

It depends on the individuals. You didn't say which way the difference was. I assume the guy is older. My daughter is married to a man 9 years older. It is working very well.