Some people use mylar or similar synthetic mouth pieces (some are even flavored). But quite often these mouth pieces are thicker than a metal bit, and can be quite bulky in a young horse's mouth. I've seen lots of these bits with chew marks on them and I think a metal bit is less likely to encourage a horse to actually chew on the bit. If you want to add flavor to make biting a more pleasant experience its easy to smear on a bit of molasses, jam or honey. If you do choose a synthetic bit and a horse chews it, get rid of the bit before there is a chance the metal core is exposed.
Because small rings can be pulled through the horse's mouth, the first bit your horse wears may not be the first bit it is ridden in. I think choosing a bit with a similar mouth piece makes the transition easier.
If you're starting your young horse in the winter time, make sure the bit is warm. A cold frosty bit is not a nice experience, even for a well trained horse. Make sure the bit is properly adjusted and fits the horse well. Too low or too high will cause discomfort and you want to encourage your horse to hold the bit quietly.