Some people like to do the ground work, long reining and even light putting to at 2 1/2, then turn away untill 3 1/2, 4. Or wait until 3 1/2- 4 and break in one go. It kinda depends on whether they're a bit of a git or not because if then are the smaller the better! Bearing in mine a clydie won't finish growing completly untill 5-6.
Our mare was backed at two (not by us) personally too young as she is still very immature and growing at 5. She has been ridden since 3.
Cairo was the same - did not realise how young he was - when it was realised he was turned away for another year.
That said, they don't seem to have suffered for being backed so young, Cairo died at 22 but not due to his back/legs.
I know a lot of them are got ready at this stage for being worked in harness and are being driven by 3.
Long reining at that age is fine but do avoid tight circles etc as they are a slow growing breed and Farrra at 4 was far more lanky and unbalanced than Chancer was when he was only 2.
Farra is rising 5, and now starting to fill out - she is a smaller clyde at 16.2 and may go 16.3. She is the safest hack, and her schooling is starting to come along - starting to balance and hold herself.
She has just started doing a few low jumps which she adores - keeps coming round to do them again by herself.
She does not carry more than 14 stone and will eventally carry 16 stone when she has had time to fill out and muscle up - probably when she is 6.
I would certainly start ensuring a 2 1/2 has excellent ground manners and is taken out to see the world before they get too big and realise how strong they are. Heavies need to have better manners than most horses due to their potential deadly size.
There are people on here with a lot more knowledge than me about them as I have only had two so far which luckily have had the most gentle (though cheeky) natures and been fantastic to ride.