Health,
My comparison between a young girl getting married and a large age differential is quite valid, as is the story I mentioned. I’m making the point that rabbonim, who pasken everything according to S.A., will not advise one to enter into an ill advised marriage, despite p’ru ur’vu and lasheves.
If based on individual personalities, a particular 40 year old man is suitable for a 20 year old woman, I don’t see anything wrong with them getting married. But that would be highly unusual, since they’re from a different generation.
Standard advice from various gedolim is not to marry someone more than ten years apart (although I imagine that it expands a bit as they get older).