If the woman is over 20, the man can be pretty much any age.
Relationships are about economics. Men are looking for sex. If a woman thinks she's getting something valuable in exchange for that, no objective person will think she is "too young" or "being taken advantage of" after 20. Otherwise we'd have to assume every 20-something gal who dates a millionaire is naive and she's letting him exploit her. They aren't so innocent.
Also, men have been marrying much younger women for millennia and it was never frowned upon until the sexual revolution and women's rights of the 20th century. It't then and there that it suddenly became gross. Before, nobody batted an eye when a rich, noble dude got a late-teen wife - rather, everyone was happy for her, provided he wasn't a notorious wife beater.