Flamingo

So basically they age slower than every pony else. Also that would mean they got their CM around 200 years old. Wouldn't that seem like a reasonable age for them to get their CM?

But don't they get their cutie marks as fillies like everypony else? They had their cutie marks for about 1500 years at least if they were 2000 years old. They had their cutie marks when they turned discord to stone (which was over 1000 years before Nightmare Moon's return), turned King Sombra to shadow (between Discord's inprisonment and Nightmare Moon's banishment). Since we don't know when Discord was turned to stone, we can't really find out their age. Discord stated that it was eons from when he last had fun, he could've meant before he was turned to ston or way before that.

What I have heard is that the are twins and that Celestia was born first instead of Luna.