I think age really doesn't matter, but for some they think oh gosh he or she could be my parents at that age. However, I have met some ladies who were 20 who acted 30 and their mentality was much older than their age and they couldn't stand guys their age since they acted too immature. It depends on how you click.
As long as the male or female is over 18 they are an adult and can date whomever they want and marry who ever they want. Of course the parents may have to get used to it, but you are not marrying the parents. If everyone went by what everyone else thinks, nobody would be married in this world.
There should be a rule, girls should never ask their friends if they should date a guy, their friends are the biggest critics.
And so are the male friends.
Go by your heart and what you want to do.