Not just start a family, but, a man can spend 5 years dating, and at age 30 meet a 25 yr old to eventually have kids with. For women, youth is an advantage in dating. For men, the opposite, which both supports and demands career success.
IMO this is probably the biggest reason for the under-representation of women as CEOs/engineers/etc. You don't have to make a lot of money as a female to attract a mate, but you're sure as hell going to have a hard time with that if you're a male.