Can someone please press Rewind?!

With age comes wisdom and maturity...or least it's SUPPOSED to.  Time has flown and as I sit here just a week away from turning 30, I find myself wondering, reflecting and dreaming.  Why do people put so much emphasis on turning 30? I've always felt like (per societies standards) folks should be married with children by now or at least very close.  I'm not there and many times I feel like I'm one of the ONLY ones.  Sometimes I feel like need a do-over.

If you could rewind the time to a certain age and do it all over, what age would you go back to? What advice can any of you 30+ somethings give to someone who has been told that it's all downhill after 30 and isn't necessarily thrilled to leave their 20s? Is age really only a number?