The implication is that the CMC had been born, but were
really young when AJ left the farm.
It also looks like a multiple of four years before the events of Games Ponies Play would have put RD soon enough before she earned he cutie mark to look the same.
In three years, the CMC and Mane 6 have not aged noticeably.
The Apple Family Reuniun happens every 100 moons and we know that AJ attended her first as a baby and has attended multiple ones since. Another occured in the same year as the events of Games Ponies Play. I'm feeling that we learned other clues in that episode, but can't think of them now.
We should be able to develop a set of equations from this that will give us an age estimate.
@dark: What'd you think of the latest
Harmony Theory? I was all, "Awwww, that's sweet. Oh, no! This is going to suck!"