It's great to feel young, embrace it. I'm also 23 and a little concerned where the last two years went. Just the last two mind, the rest I can account for.
I'm 22 and will be begin mine next month. It feels wierd doing a PhD at this age. I don't know what to expect in the years to come but i shall embrace it regardless, but i guess its something to be proud of.
30, I was 29 when I started and 28 when I first met up with my potential supervisor, who said he only took up experienced professionals as PhD students (never under 28). The field is not a scientific one, though, but a very applied one (Management).