A newborn foal will get some antibodies to fight off infection from its mother's milk, but like human babies and puppies, they may need other vaccines and regular deworming. Also, just like us, our immune systems get stronger with age, a good diet and a healthy environment.
Hoof growth usually slows down as horses get older. My horses are twenty something and get trimmed three times a year. I have a couple of clients with horses in their thirties. I haven't trimmed them since September, and still didn't need to trim them when I looked at them the other day.
Hooves and a Healthy Immune System
A healthy diet and exercise are not only essential for horses' hooves and immune systems, but for overall health. Horses that are overweight or lack important vitamins and minerals in their diet probably have weaker immune systems and hooves.
Horses are flight animals and they like routine. Even the slightest change can cause stress. This can be anything from changing their hay or feed times to moving your horse to a new location, bringing a new horse into your herd or having a scary plastic bag blow through their field. If you're moving your horse or trailering them to a competition where they will be around other horses, it's a good idea to protect their immune system and stay up to date on all their vaccinations and deworming.
Learn to Read Your Horse's Feet
Have you ever noticed the horizontal lines around your horse's hooves? I don't just mean the line in foals that separates the baby and adult hoof, or the line that drops down at the heels on horses who have laminitis. Some call them growth rings, others refer to them as stress lines, but most horses have them.