One who has amatory connections with someone much younger; indeed, too young to be having any at all. Synonym: cradle robber (q.v.)
Under the word Under-age Webster 1913 has this to say, from his illustrious but shocking namesake the playwright:
I myself have loved a lady, and pursued her with a great deal of under-age protestation. J. Webster.
I have also heard this as snadle cratcher.