I think the best age is when the person feels ready to bring about this major change in their life. Marriage requires a big adjustment in one's life. It's best if the person is mentally ready to make those adjustments. That mental maturity can come at any age.
There is no best age, it all depends on the individuals. It also doesn't have to be a man or a woman, as thankfully same sex marriage is now legal. In general, I would say late twenties to early thirties.