what bit???

Chika1235. You have got some really good advice, for goodness sake don't ignore it! If someone is telling you that it is ok to start a riding horse at 18mnths you need to tell that person to go to hell because they're a fool.

if this person is really set on breaking this young there isnt much anyone can do to stop the op. all we can really educated them. There is a whole page of posts about the exact same thing, it gets annoying, however i do agree with everyone else.

You should use whatever bit the professional trainer that is helping you suggests. The fact that you had to ask says you're not ready to do this on your own.

I like a jointed Dee ring snaffle. As for breaking it use to be the norm to break a horse on his second birthday and I feel people leave the breaking until too late. TB are broke as yearlings to get them ready for racing as 2 year olds. The arabs use to break their horses as yearlings and when I was young and light we broke the yearlings.
I broke my big jumper on her second birthday.
I feel leaving a horse until 4 is getting too late, they are too set in their ways and as some put it tooo resistive

I think 18 months is really young too, unless you are only doing ground driving and things like that.

As for a bit, I recommend a flexible rubber snaffle. That is what 3 of my horses started out with and I find them to work as well as any metal bit. The are also really soft on the mouth IMO.

I would suggest putting just a jointed or french linked loose ring snaffle. The lighter you start them, the lighter they will be with a bit. I would never ever suggest starting a horse on any form of gag bit or shank because it will take a lot of effort to bring them back onto a softer bit; myler's I have also found take a much longer amount of time to get a horse used to, none of mine like them and are very confused by them. Are you planning on riding the horse now? I would start doing a lot of bitting and lunging with a saddle on for a few months - I wouldn't ever sit on an 18 month old horse; wait til he's at least 2 - waiting til 4 to ride isn't necessary, but 18 months is just too young. A lot of babys will be much better about your concern of him running off with a soft bit if you begin bitting him before you ride him. Either saddle him up and tie the reins with contact to the stirrups and get them the feel movement of the bit with their natural movements or you can do it without a saddle. I have a just 3 year old who I take a pair of split reins, put it inbetween their front legs and then tie the reins just where they have slight contact on their back. You can do this for a while til they get more of a feel of the bit contact and progressively add more tautness to the reins. My horse has a great headset on her own by doing this and is very light on her mouth.

I also have an 18 month old QH that I want to start undersaddle and i use a full cheek snaffle. and as for maturity it really depends on the horse, my guy has had a saddle and bit on and i have started teaching him to stand for mounting, i have not yet rode him but he could really care less what is on him or what i do to him. so it really does depend on the horse he will let you know if hes ready.

It's not mental, it's physical. They are not done growing and their growth plates have not closed. You can cause long term back, hip, and joint problems starting at such a young age.

It's like saying "My eleven year old is mature and hit puberty, so she's fine to get pregnant." Just because they CAN do it, does not mean it is safe or healthy for a young, growing body