Normal would I think the answer as something elementary-aged.As the colts and fillies look so young and child-like,with most of them getting a cutie mark on such a young age.
However,once I found this on the mlp wiki,I started to doubt myself.
"A "cute-ceañera" is celebrated by those who have recently gotten their cutie marks. The name of the celebration is a play on quinceañera, a celebration of a girl's fifteenth birthday in parts of Latin America."
(http://mlp.wikia.com/wiki/Cutie_marks)
Which could actually make sense.Come to think of it,let's take a look at the fillies and colts.Seriously,what they do seems to exit what a real elementary-aged kids could.Like car-driving,making a car,etc.Also,not many people can learn their destiny within 12 years old,right?So could it be possible that our fillies and colts could be somewhat like,a young adolescence,middle-school aged?
Now let's take a look at the mares and stallions.They may look like teens,but it seems that beside of fashion and mane-style tastes,teens and young adults sure don't share any differences.(We can see that in Call Of Cutie,Cheerlie don't much different in her teen years than recent.Tho the show didn't specify which stage of teen year was she in.
The mane 6 may look teenish.But in fact,while Twilight being Celestia's student and in research,I could assume she's University-aged,or even on her stage to getting Master or PhD?Also the mane six all seems to have jobs,and it seems perfectly normal(Fluttershy's brother got blamed for not getting a job).It's obvious that the mane 6 aren't teens.
It's is said that horses grow rapidly once they reach their puberty.Can we assume that ponies,also,are like so?Can we assume that the CMCs are actually teens,rather than children?And,thus,can we say that ponies usually get their cutie mark on teen ages?What do u think?