It's because people are scared of ageing. They are afraid of losing their youth (looks, physical energy, etc) and also afraid of the social seniority that comes with it. Every year you get closer to a mile stone (18, 21, 25, 30, 40, 50, etc) and as morbid as it sounds, one year closer to dying.

When I turned 18 I suddenly became very age conscious because I became a legal adult that was allowed to do everything, yet I still felt (and still do) like a young teenager, I now dislike my birthday, although I've learned to just accept my age and not really think much of it, but I am still rather age conscious. It is just a number, but it's one that helps define us I guess.

More power to you that you embrace ageing and see it in a positive light, though.