I do celebrate my birthdays, but usually on my own terms, with others it would be going out to a restaurant to eat, hanging out with each other at my place or their place, or receiving gifts. I could see that when someone gets much older (like over 40-50) there is just nothing really special, since it's just another year closer to retirement and eventually, death (mortality).
Now for the younger age group, like children or teens, I could see it being more meaningful as there are more exciting milestones in life. (13 is when someone is a teenager, and often starting high school, 18 is a very special age as it opens up a lot of rights since someone would be considered, at least legally (US), as an adult, as well as going to college, and 21 drinking age (US), etc.) However, after a certain age it all becomes pointless as there are very few milestones related to age unless someone finds something really special like the age at which they are considered a senior, or special anniversary/occasion.