I'm a 27-year-old woman, and I've been dating a younger guy for the past few months. We are definitely in different places in our lives, and I'm not sure how to reconcile our different experiences with the fact that we have fun when we're together. How young is too young?
You don't say what the age difference actually is between the two of you, and I'm hesitant to quantify something that can cause a lot of hang-ups and unnecessary strife. It's an issue that can even become a drawback of online dating because you can search for potential mates within a specified age range, but someone a month (or a year!) outside that range who would be great for you doesn't show up.
It's also one of the reasons that many people, both men and women, lie about their age, telling themselves things like, "I may be 35, but I act like I'm 28," or, "I may be 40, but I look younger so no one needs to know." Dishonesty aside, looking at where you are in life and your common goals and values is a better indicator of dating compatibility than a number.
Society continues to draw a distinction between couples where the man is older ("normal") and couples where the woman is older ("worthy of lots of obnoxious comments"). You're likely to get all sorts of scrutiny from nosy friends and relatives when they find out how old this guy is, especially if he's in such a different place from you that, say, he's still an undergraduate while you've already completed grad school, or he's still living with his parents while you own a condo.
If you're comfortable not settling down right now and possibly not ever settling down with this guy in particular, then the fact that you're having fun overrides any other drawbacks to the relationship. However, if you are looking for more permanent arrangements for your romantic future, then you shouldn't be spending time with guys who aren't suitable for you in the long-term, whether that's because of their maturity level, religion, or lack of geographic proximity.
If you're trying to decide if this guy could be "the one," try to feel out his "seriousness potential" and be honest with yourself about what you learn. Also be aware that he might realize what you're doing, and your fact-finding mission could scare him away before you even decide how you actually want to proceed. But, since you're having fun and you don't reference any other issues beyond age/experience, give yourself a break to enjoy this experience.