If you have a created horse whose first year of racing is at the age of 4, does that mean he has a few years of peaking or is he already over the hill? Or is his peaking time shorter?
I imagine that the horse is just like any other horse in the game, even if he doesn’t get generated until he’s a 4yo. So, potentially, he could have peaked as a 2yo and 3yo based on his stats, and when he starts racing as a 4yo he’s on the downward slope.
However, I feel like there’s a lot of late bloomers, and for many horses late 3yo and 4yo years are still in their prime, so you could potentially get part of their prime years. 4 is perhaps the oldest I’d claim a horse, though.
That’s just me, though.
I can’t remember 100%, but I believe randomly created horses are guranteed to at least have part of their peak running time ahead of them, when created. Obviously if you create a random yearling, it’ll have its whole prime ahead of it, but if you generate a 3yo, the script will ensure that it didn’t peak from may-sept of its 2yo year, for example.
I haven’t actually looked at the code yet, and I’m still half-asleep, though, so no guarantees. 
Thanks. I’m worried because my best breadwinner was generated as a 4yo and I’d hate to see him slumping so soon after his successes.