Horses are not fully mature until the age of four or five, but most are started as working animals much younger, though care must be taken not to over-stress the "soft" bones of younger animals. Yearlings are generally too young to be ridden at all, though many race horses are put under saddle as "long" yearlings, in autumn. Physiologically young horses are still not truly mature as two-year-olds, though some breeders and most race horse trainers do start young horses in a cart or under saddle at that age. The most common age for young horses to begin training under saddle is the age of three. A few breeds and disciplines wait until the animal is four.