Getting a yearling used to having a bit in it's mouth isn't really too young. I put a bit in my mares mouth for the first time at 7 months, just let her feel it for a few minutes at a time, a few times a week. Most of the trainers I've worked with start that at about a year old. Breaking fully to ride, too early. Bitting/Mouthing, not too early.
That being said I would start with any kind of snaffle. Be careful with rubber, sometimes it is too thick for a young horses mouth, or a smaller horses mouth. You may have to try out a few different styles until you find one that your horse really likes, but the gentler the better IMHO.