Oh I know the feeling. 30 is nearing for me too. Oddly enough though this last year or so I've become less freaked out about growing old. I think it was when I discovered my first grey hair (yup, a grey hair at 27. I wasn't impressed) it then hit me that 'this is gonna happen. Deal with it.'
So I've accepted that I will grow old, but truthfully I'm never going to grow up. You're as young as you feel in your heart, and my heart is always going to be 21.