I think you shouldn't focus that much on their physical age but rather on their spiritual and intellectual age. Someone who is younger than you can be as mature as you too, but of course that's not always the case. I think it's a good thing to discuss with your mom, after all you will need her blessing one day haha
To answer your question though, I'd say maximum 10 years older. The most important thing is that you get along and you feel spiritually and socially connected.