From the experts:
Probably due to its nursery rhyme association, Jill has the perpetual air of a rosy-cheeked tot -- even though it is one of the oldest names on the roster, a medieval variation on the Roman Julia. The pairing of Jack and Jill to connote a generic boy and girl goes back at least to the fifteenth century. But can knowing Jill's history keep it from sounding like a cute mid-twentieth century invention? There may be some possibility of Jill making it back up the hill.
Jill entered the Top 100 in 1957 and held that position until 1983, dropping off the list completely in the year 2000. A notable present bearer is Jill Biden.