That's just some half-assed lame custom that has no particular justification behind it. The age of consent is usually no more than 18 pretty much everywhere (at least in the US, where most of the HN posters seem to be), so I don't see any problem with a 50 year old guy on a date with a 20-something.

I figure anybody complaining is probably just jealous. But I'm close to 50 than 20 myself, so I'm not exactly unbiased. :-)