[Oldexpat]

That is the old Chinese formula. It makes sure the man is mature and has enough money to support a wife and family. And makes for a lot of rich widows.

[WellyP]

1/2 your age plus 7 years, give or take a couple of years seems to be the lower limit. At let's say, 65 a man marrying or in a long term relationship is likely to live 15% longer than average if his lovely younger partner is 10-15 years younger than he is. Those numbers seem about right but I think it applies better to very healthy active men (and women) not your average couch potato!