*Ranch Worker Raises her Hand*
The life cycle of horses:
Horses grow up pretty quickly. Little filly Epona in OoT struck me as "not quite a year old yet, maybe a year" - a horse is an adult (young adult) at about two years. 1-2 years is when most people start training them. (Well, you do train them to not have certain bad habits from birth. The barn I work at temporarily housed a mother and young colt and the owner of said instucted me not to pet the colt or hand-feed him because at that young age, they can learn to bite if you mess with their mouths too much). My sister had a colt that begun saddle-training at about a year and half...
Seven -eight years old for a horse is rather young-in-the-prime of life. Showing Epona in OoT after seven years fully grown and in full muscle was something the game creators "did right." I imagine, depending upon the invidual and their obedience/calmness, that around eight years old is a good age for a war-steed.
Horses get old in their late 20s... and live around 30 or so. 40 is a good long age for a horse.
And, as magificent as they are, horses are all poo-factories. I'll go get my pitchfork.