In a few days I'll be 48.
When I turned 40, it wasn't such a big deal. We had fun party--with a Birthday Throne--which I enjoyed very much. Turning 30 was much more traumatic for me. I think turning 50 is going to be like turning 30. I have a vague sense of ... dread? Sort of like I'm like Tolkien's painter, Mr. Niggle, and I've forgotten to pack something.
On those days when the glass feels half-empty, I remind myself that I plan to live to 100 (Mark will chime in here and say something about exercise and eating right), and that the years so far have been mostly fun.