horse hooves doubts

Question
hello madam,
1)at what age, the horse hoof will be fully grown??
2) horses with bigger hooves tend to develop more resistance. is it correct? if yes, may i know why??
3) why ex-racehorses, especially the stressed out ones, tend to develop more resistance, is this due to growth in the hoof size. if i'm wrong, please correct me!!

Answer
1. the horses hoof will always grow but width wise it will be full size at age 4
2. it doesnt matter what size hoof it is is they can all have the same resistance. some horses may have better resistance  because their hoof is harder it just depends on the horse and the terrain it grows up in
3. again it just depends on the horse.