cradle-snatcher someone who has a romantic or sexual relationship with a much younger partner (thefreedictionary.com) You could try forcing a neologism such as juvenophile, but any term with this general sense is likely to be derogatory and/or humorous.

Is it weird to date people younger than you?

Its pretty common to date someone whos a few years younger or older than you, and often the age difference is no big deal. Sometimes, maturity levels match, even when ages dont. But when the age difference is bigger, there are other things to consider. Age isnt the only factor affecting power.