Good explanation of the stuff.

From my understanding, the age required to enlist in the Trainee Corps is twelve years old; while the exact age itself wasn't specifically stated, I've theorized it from the following:

Eren's age in Year 850 is listed as fifteen.
The breach occurred in Year 845, making Eren ten years old when it happened.
He did not meet the age requirement at the time, so he had to wait a few years.
The 104th Training Corps did not come to be until Year 847, and that group graduated in Year 850.

I think that should mean our characters should be between the ages of twelve and fifteen / any upper limit instead of 10-15, so that there isn't any conflict with the designated age requirement. Like it'd be weird if we had a 10 year old character who would technically have to wait two more years in order to join the next Trainee Corps.