I don't think it's the fear of getting older because lets face it, it's better than the alternative but for me it's leaving your twenties behind. This decade is supposed to be when you are carefree and fun loving, instead it's about clambering onto the property ladder and paying bills. Getting to thirty means you have survived the most financially difficult decade and realising that you haven't been to Glastonbury or driven across route 66 in a Cadillac as you always hoped you would can be difficult, I imagine. Yes there's always some annoying twenty six year old (he he he) at hand ready and eager to take the mickey, and this will be true every decade to come unless she who will not be named pushes her luck and does not survive!
Getting older however will not stop me doing what I want. Even if I'm driving that Cadillac when I'm seventy nine!!