I suppose when that change happens along side a bunch of other changes (i.e. when you’re leaving school, etc.) that’s one thing.
For my situation, my friends and I have known each other since we were 10–12ish and are all 30+ (yes! there! I admit I’m 30-something :p) and are now spread over 4 different countries, so we’ve already seen a lot of changes. But for some reason, the kids thing makes a bigger difference than most other things (relationships, jobs, buying houses, etc.).
I don’t think I’m jealous, but I must admit, sometimes I wonder if I’m missing out – doesn’t last long and then I remember that it’s a very big and very long term commitment that I might be better off without.