A horse can mature in size normally at the age of 3-4 years old but can take longer depending on breed and previous nutrition. A horse can mature mentally anywhere from age 2 to age 6 it depends on the environment and training and the horse itself. A mare matures sexually at about 3 years old, but its bad practice to breed before age 4- pregnancy can be damaging to a young horses body and maturity as well as being very stressful both physically and mentally resulting in miscarriage, early foaling and bad parenting.
A stallion can mature as early as two years old (depends on the dropping of testicles). A colt can exibit "studly" behaviour at age 2 but not be physically mature to produce offspring. Typically 3yrs is mature sexually for a stallion.