18-40 is the proper answer.
18 to be legal.
40 is good if you're 18, 23,28,33,38,43,48,52,59, etc.
You can kind of up the 40 . when you're over 50, though ( unless you're rich, then you want 30 ish).
most important factors are she has no kids, don't marry her if she does....at least get a pre-nup if you have big $$$, and insist on that stupid move.
40 is when they start to go bad, IMO !