Why is it that we seem to mark some birthdays as more special than others? Place more significance on them? Dunno really, not even sure every culture around the world does this... And those that do, don't necessarily have the same milestones!
I guess my own mixed cultural background comes in to play here...
The first big milestone I remember - yet didn't celebrate - was 15. I was living in Mexico then and the "Quinceañera" was a big deal! But I mostly saw that through a friend's birthday two years after mine, as my family was on holiday in the US Southwest (fabulous summer camping in the "4 Corners" area!), and I was actually in Vegas for my birthday! Perhaps if we'd been in Mexico instead of out of the country there might have been a "Quinceañera" flavoring to the celebrations? Your guess is as good as mine! And it's not like many of my friends would have been in town, they were all off on their own summer holiday (international school).