In horse isle, there are two game modes. One is Eternal, and the other one is Life Cycle.
A life cycle horse
Eternal[]
Eternal is realeased on 2010
In Eternal, horses don't age, which means they will NEVER die. This also means they will not breed, so stallions and mares basicly don't do anything in the game, only by changing their appearance. You cannot geld your horses and the breeding facility of ranches will not be available. You can only bring 5 horses with you unless you have horse barns in your ranch/join a club with horse bonus/have an esroh pitchfork or any order things that let you own more horses.
The Age Recommendation for Eternal is 11+
Life Cycle[]
Life Cycle was released on 2012. In Life Cycle, horses DO AGE, and they will die in a certain age(25.5 years, approximely a real year), and when they are around 20, they can no longer participate in arena and can no longer breed and are known as "Old horses". You can breed them by finding breeding facilites, check out other articles for more about breeding. You can bring 10 horses, not including the horse barns.
Horses in LC usually have a better stats and personality that Eternal, to get a good horse, you can breed 2 horses with good stats and genes, breeding facility can be expensive or cheap, according to the stallion's stats and personality, if it got good stats and personality, it can breed for very high price! If it's not that great, the price will usually be cheaper.
There are more limits if you are a non-subscriber, because you can't breed your mare horses with your own stallion horses. The only way to breed your horses is to go to someone's ranch, pay for a stud fee.
Because you can breed horses, there will be 25% less chance that you can find wild horses in LC than in Eternal. (In fact, some people don't play life cycle just because there are less wild horses)
The age Recommendation for Life Cycle is 16+, however, many younger people ignore this rule.