As part of a young generation, me, my sister, my friends, and everyone else I know can confirm that Seinfeld certainly does hold up.
Edit to give a bit more context
My sister is 18, my friend is a few younger than that, my uncle is in his 20s, I'm an age, we're all within the "teens and 20-somethings" demographic. I haven't watched a lot, but I love what I have seen. It's my uncle's favorite show (well it was, it's probably beaten by Game of Thrones at this point :dodgy: ) as well as my friend's, both have watched the whole series numerous times, and my sister watches it plenty (if a bit less than the other two).
Maybe we're just weird.