When it comes to celebrity relationships, is age really just a number? According to a recent study, the less of an age difference between partners, the more likely a relationship will last. While troves of Hollywood men have been notorious for testing the limits, A-list ladies have also given younger companions a shot. Kate Hudson was linked to Derek Hough, who is six years her junior, and it's impossible to forget Demi Moore and Ashton Kutcher's marriage, which lasted for eight years despite the 15-year age gap. Read on to discover the most memorable relationships between famous women and men who are at least five years younger, and then find out which celebrity couples have stayed together the longest.