As it was pointed out to me previously, yearling is not a very good term to use for teens because its technically lower than a foal. I stuck with it for a bit just to see but no, I think it's too confusing in the end.

So here is the new ages based on this:
Newborn = Foal or Suckling or weanling
1 year old = Yearling
(( Now this is where we start fudging the ages because its not really clear how pony aging works.))
Childhood = colt(m) filly(f)
Teenager = New Stalion(m), New Mare(f) (Ponies in their "Prime" )
Adult= stallion(m) mare(f)
After childbirth = sire(m) dam(f)
Elderly = Old Stallion/Mare or "Geriatric" Stallion/Mare

So two things are changing, yearling and my use of the word foal