The answer to this question is rarely, and that's because younger guys generally suck. They're immature, always think they're hotter than they actually are and really just don't know what the they're doing. That being said, there are exceptions to every rule and the younger guy situation is no different. There are only a few reasons it's acceptable to date a younger guy and here they are. You're welcome.
1. If he's not that much younger.
The difference between 23 and 24 isn't a game changer but 23 and 30 gets weird really fast. Kris Humphries was four years younger than Kim and look how that turned out. RIP Krim. Kanye is 37 and he and Kim are thriving like never before. Go figure. If the guys is within a year of your age, you're not a cradle robber but if he's over three years younger, things get a little tricker. The age difference becomes less important as people get older, but let's be honest, that's just an excuse to date older, richer guys who are much older and much richer. The logic doesn't apply in the other direction.
2. If he's incredibly hot.
Samantha dated Smith and it worked out until she dumped him for herself, which also worked out. Does anyone blame her? No. He was a male model. You don't pass that up. Similarly, Demi snagged Ashton Kutcher before he put on the black turtleneck and pretended he was Steve Jobs in that really awkward movie. Why did she do that? Because he's hot as fuck and she just freed herself from Bruce Willis who is bald.
3. If you're in college and bored
Everyone knows college isn't real life so real rules don't apply. By the time senior year rolls around for the average college betch, the male market is not what it once was. This is because more than half the school is younger / dumber / more annoying than you are. In this case, if you want to start hanging out with some muscular, tall freshman on the football team, who's judging? Only jealous bitches. However, it goes without saying that this 'relationship' is a joke and should be treated as such.
Really, that's it. Younger guys can be fun hook-ups, but when you're 27 and dating someone fresh out of college, you have to look yourself in the mirror and reassess some things.