A good time to get married from my personal view is when a person is fully mature, emotionally, physically, mentally and spiritually. That can handle the stress of being married and eventually having kids. They're more selfless than selfish, so they keep the relationship together instead of going through a bad marriage (and to avoid divorce). The man should have a stable job and can provide for the family.
People should have enough life experience and be smart because you pass on whatever smartness or not-so-smart habits and attitudes onto the children.
And people mature at different rates, for me personally I'd be around 27 because I know it would take me a little bit to mature more to the extent that I can handle such responsibility of being married and having a family. For some it comes earlier and some later.
A better question would be "what are you doing to prepare for marriage?"
"what can I do to be a great husband/wife?"
etc