if you mean literally back as in sit on and poss WALK around a school or the like then turn away, 2 1/2 but actually break 3 1/2 imo. Partly my background with big horses, it's not a bad idea to get some things into their head while they're smaller! Although with those types they wouldn't be working until 4 at least.
I just paid £100 to have my colt gelded but that was a half price offer day at the vets! I got a couple of quotes and both were for over £200.
I would back a horse and get it quietly hacking at 3 then do it properly at 4.
My colt was £180 to whip em off..backing would be dependant on the maturity of the horse and the time of year..when my now gelding is 3 year old it will be June so dependant on maturity he will be saddled and sat on basic work up to winter then left over winter then started up again come spring/early summer but will purely be dependant on how mature he is!
I have very lightly backed my 2 and a half year old colt this year just to get him used to the idea as I'm keeping him entire and don't want to try it when he's twice as big next year. He took to it very quickly after a few days allowed me to sit on him with a bridle and saddle and walk around the yard for 5 minutes when I got on him. After a week or two I turned him away (still having a lot of ground handling) ready to back him next year when he's 3 - 3 and a half depending on how well he has matured throughout winter.