I do not think "important" birthdays should be decided by the number of fingers we have and the fact that we are neither Babylonian (only their 60th really counted) or Saxon (multiples of 12), so I have come up with a new importance scale, which better reflects the changing nature of time with age. Simply use the square numbers.
1 - Your first. You won't remember it, but trust me - there'll never be a time when you're as universally loved. Look back the pictures. Even belching made everyone happy.
4 - The first one anyone's likely to actually remember, even if second-hand from your parents' stories of it. You are probably beginning to develop some kind of rudimentary personality, which obviously should be crushed at the first opportunity. In the case of boys, you are probably still at the age when you obey your mother when she tells you to walk round puddles rather than through them. This changes, obviously, on first contact with other children. Peer pressure is an instant, and terrible thing.
9 - Right in the middle of school, when fun is still paramount but the pressure (even if you have an 11+ exam in your area) has not yet begun. An idyllic time. Slides are great. Jumping about is great. Getting sent to your room during your birthday because for some reason your parents won't let you win every game at your own birthday is... well, hey - no one's perfect.
16 - In many societies you become an adult. You certainly get certain responsibilities. By now you may have discovered the opposite (or same if that's your inclination) sex, and from here on will turn into a babbling wreck. If you are one of those really irritating people who just appear on the stage with some social skills, this is when you're at your peak, mainly because the rest of us are looking at attractive young things in the girls' school opposite and going "bwaaaaaaaaaaa" in their general direction. Still, we didn't have responsibilities.
25 - If only we'd known, we'd have properly enjoyed this age. Or at least I would. A job, a place to live, people around me, still not much in the way of actual responsibility, and the whole of life still lying ahead. I had started writing by this age. We had parties. We were adults, at least in name.
36 - A family. Grown-up stuff. Marriage, child, and yet still not a lot in the way of aches and pains. A chance to consider what has been done, a chance to look back and forth, six square birthdays down now but still probably as much life ahead as behind. A good mid-life birthday to relish.
49 - Family growing up. Maudlin feelings of what has not been achieved since number six, but with square birthday number seven in full sway, perhaps a wise acceptance of what has been achieved, especially in the way of the aforementioned family growing up, and no one having exploded, but perhaps a certain focus due to the knowledge that of your square birthdays, there's a fair chance you've only got one or two more of them. In theory there should be some time spare. Use it.
64 - I'm not there. I don't know. I'd hope to be retired, going for nice walks in the country. I'd hope to have family Christmases still, and family get-togethers to find out what Number One Son is up to, and sit with Mrs Me raising eyebrows about how we were there before, and how he'll learn.
81 - The distant future, still. Who can say.
100 - Unlikely. Not necessarily desirable. My mother once gave a fiver to a woman begging in London, and this woman replied with "Bless you, may you live to be a hundred." My mother replied "My dear, never wish that on anyone."