War Horse is a coming-of-age story told at an accelerated pace. Joey's turbulent teenage years occur when he's only six months old, "wild and vicious" (2.9) and listening to no one. If he could talk, we wouldn't be surprised to hear him say "you're not my father!" to Albert's drunk daddy and stomp off to his barn to sulk in a haystack.
Joey matures from pony to stallion in a matter of years. With Albert's guidance, Joey learns discipline on the farm field before braving the battlefield. He lives through the death of his close friends, makes a few mistakes, achieves personal success, finds what he is good at, and is rewarded for his accomplishments. These events are the hallmarks of any good coming-of-age story. All that's missing is an awkward first kiss, and this would be a by-the-book young adult drama.
Even though Joey's life journey passes by so fast (and, um, he's a horse), it's no less relevant to many young people who have gone through similar experiences in life—especially during times of war when everyone is forced to grow up fast.