It's a big deal for young horses when they're first bitched and mouthed, so get started as soon as possible (within reason). A bit is a foreign body in their jaws that they can't get rid of for them.
There is a lot of change in a young horse's mouth between the ages of 0 and 5, and even later in some situations. Teeth erupt and baby teeth fall off, much like they do in human children. One thing that must be done before any biting or mouthing is attempted, and obviously every few months during the horse's life, is to have the horse's teeth examined by a trained Equine Dentist.
Mouthing bits for young horses is a fantastic place to start because they are meant to be comfortable while also providing something for the horse to 'play' with. For this reason, many of these bits for young horses include 'keys' or something similar attached to them.
These'mouthing' bits should be gently introduced and left in the mouth of the young horse for short amounts of time initially, eventually increasing to longer lengths of time. At this point, do not connect any side reins or anything similar. It's critical not to leave your horse alone while it's being mouthed.'
Consider the breed and mouth conformation of the bit while selecting one for your child. Bitting a young horse must be done appropriately from the start, so it must be at ease and content with your actions.
As long as you're both comfortable, actually putting the bit in the horse's mouth shouldn't be a problem. Don't rush into the stable and slam the bit into his teeth; neither of you will benefit from it. Instead, walk up to the horse and put the bridle on as usual, but wait until the horse willingly accepts the bit into his mouth. With a well-placed thumb, you may have to coax him to open his mouth but maintain everything quiet and without applying any pressure.
While fitting, make sure the mouthing bit is comfortable in the horse's mouth and that no pressure is applied to the bit - he is still a long way off from this stage. If the horse is reluctant to accept the bit, spread something tasty on it to persuade them to open their jaws and accept it. All of this should be carried out in the quietest, least stressful manner possible.
Check for symptoms of discomfort and rubbing on the sides of the mouth every time the young horse wears the bit; if this occurs, you may need to reconsider the size or shape of the bit.
You'll quickly observe how the horse reacts by chewing and fiddling with the bit keys. Don't be concerned if this does not occur the first time you mouth your young horse. A loose ring mouthing bit will encourage him to do so, and they will develop a soft, wet mouth as a result. There are many different varieties of mouthing bits available these days, but they all function in the same way. Remember that the young horse will view the bit as a foreign entity in its mouth, so make certain that the one you use will not give any discomfort to the animal, as this could lead to far more serious issues as you gradually urge the horse to accept more.