Sometimes I wish I could be anonymous.
Anyhow, I have a different perspective on this.
In school, there are usually a handful of guys who get a reputation for dating undergrads. And they get teased for it. It's mostly good natured ribbing, nothing too serious or indignant (at least when I was in school). And I hear you all about the "different stages of life" blah blah blah.
However, my view now is: WHO CARES.
If you want to, go for it. Seriously. You'll likely have a great time. Don't rationalize yourself out of doing something if your impulse takes you there.
This whole "different stages of life" also presupposes a lot -- that you guys with a few years' experience are that much more mature and "adult" than the undergrads.
And the truth is, you're not. You think you are, but you're not that grown up yet. Especially if you're still single or not yet married, believe me, you're not as grown up as you think.
Looking back at myself and a lot of my fellow classmates and peers, at least amongst the folks with no children, the only substantive difference between us and the undergrads was a bigger bank account, and a taste of the real world. But emotionally and mentally, we were still wrestling with many of the "who am I really?" identity issues that the undergrads faced. A lot of the base insecurities all the way from childhood remain. Yes, you have more exposure to the world, but are you really sure you know that much more about yourself as a person than you were in undergrad? A bit more perspective and maturity perhaps, but you're not *that* far removed from your undergrad days. You have the same kinds of neurosis, inferiority complexes, self image issues, parental issues, etc. that you've had since you were in junior high. You want to fit in, you want others to really like you, you really care what others think of you, and so forth. It's all normal. I've seen most people change the most after they've gotten married, and especially after they have children. Or something monumental in their lives - a death in the family, or some life-changing circumstance, or simply age (you won't be the same person in your late 30s or 40s).
A lot of MBAs seem to be torn between trying to relive their youth in school, and being in a huge rush to be "adult" and older than they are.
Most of you guys (and gals) are in your mid- to late-20s. Don't feel like you're in a rush to become an adult. Because when you're in your 40s, you'll wish you were a lot younger. Consider it an opportunity to do what you can't really do and to be what you can't be when you eventually become domesticated.
Not saying that everyone should date undergrads haha (or any undergrad) but don't presuppose that they're off limits or anything.
Also, if you are to date one, it's not because they want you to join them at some frat/sorority party or anything. They are hoping you can offer what the undergrad guys can't really offer. Which is a window into your world. Going to a nice restaurant. Going to nicer clubs. Maybe even something more cultured. They want to feel a little more adult, and being with you will make them feel that way. Of course, guys are just guys (whether a college dude or a grad school dude) - the desire and goals are the same - just the path to get there will change.
As I said before, you aren't that much more mature in b-school than college (not as much as you think), but that illusion of maturity is what they are looking for in you. Of course, not every undergrad will want to date you (some will simply think "ewww. Old" but some will. Note I'm only speaking about guys - no idea how it works with women MBAs dating undergrad guys, or the gay scene (which is a whole other ballgame, no pun intended).
And if you're worried about "MBA network" or "reputation amongst your classmates" -- believe me no one will really care as much as you think. In fact, as time goes on quite a number whether they admit it or not would've wished they went after undergrads too when they had the chance.