We bit all the yearlings for the sales (TB) start off with a little soft rubber bit and leave it on in stable then work up to a vulcanite one then straight bar metal. A riding horse yearling would be fine in small rubber one and led off the headcollar.
I just usually pop it in, leave them a bit then lead them around once they're happy. They usually go through the whole mouth wide open trying to spit it out, but then are ok after a bit. I find leading them about gives them something else to think about rather than them just standing in their stable trying to get rid of bit!!