It is assumed that by the time you hit 30, you are mature enough to at least pretend that you are a functioning adult. All that late night partying isn't for your anymore - or your liver! You just have to wind down a little and maybe do some more... profound stuff.
Or you can read all about the differences between the pre and post 30 eras in this comic. Do you see yourself in these pictures? Are you some filthy under-twenty-een? Well, look at your future, then!