I think as long as the relationship works for the people involved then age isn't an issue. That being said, i have always seemed to gravitate towards Doms who are at least 20 years older then me... actually come to think of it I have gravitated towards older men in general be them Dominate or vanilla. Seems the average age of the men i have respected, loved, and enjoyed the most have been closer to my mom's age... lol.
The reason is simple... i need someone that has the life experience i don't have as of yet. i like men who are confident with themselves (confident not cocky there is a different) and who know what they want, have achieved it on some level, and know how to get what they are still seeking. It isn't that younger men don't do that, some do, but the maturity is often times a overall factor.
That being said, age alone can't be a determining factor. I know some 60 year olds that act 12 and some 21 year olds that seem 40...
As to online... age doesn't matter for a friendship. At times it is possible to tell age by the choice of words, syntex, etc. But, there have been times, even on this board, that I have been surprised by someone who i thought was older being much younger and vice versa.
So... there are my early morning ramblings on the subject....