Maturity, spiritually & emotionally are vital. The rest is a toss up really...I've observed EVERY possible combination.
One couple I know fit perfectly and the husband is 10 years older than his wife. You'd never know by watching them.
Another couple the wife is four years older but, her hubby only just turned 21 and was lacking that hmmmm non-momma's boy vibe DISASTER. She's SOoooo unhappy :-(
It's different in every case but, I certainly don't think it's okay to eliminate a possibility completely because of age.