The number of people older than you never increases, amirite?

Until you die.

What about the ladies who celebrate birthdays every decade or so

The number of people the same age as you always decreases

This isn't true, due to special relativity, the faster an object moves relative to another object, the slower that first object experiences time.

So if a person moves very fast, "the number of people older than you" would most definitely increase.

Scott Kelly (an astronaut) who is also a twin jokes about being a microsecond younger than his twin now after traveling in the iss for a year.

Unless dreadful weather vents...

Unless you go to space.