hmmmm I have had 2 babies so far and Mae is having a 3rd for me - SO I guess I have a different idea
Do as much as possible with them while they are little and tractable

Dont pound their little joints for hours on end - dont lunge until they are dead - but fuss with them a little every day
I started fussing with mine when they hit the ground - playing with their ears handling their feet - anything like that brushing and grooming when I brushed mama
Rose went on light trail rides when she was still following her mama - Wish I would have had the same opportunity with Dan - I will be doing this with Mae's baby - even eventually ponying her (if I can get the others to cooperate) Mind you mine are in the woods, on a gravel road and over varying terrain - I think it is good for them to get out and about
I started throwing a saddle pad on their backs before they shed out their foal coat (just for a second - tons of praise) and then here and there later on. I teach them to give in to pressure, to stand tied (we dont really fully "tie" until they are over 6 months - they dont have the capacity to retain that before 6 months)
I also spend a lot of time loving on them gently pulling their ears - getting them used to clippers - and hugging their heart girths - I do this to get them used to pressure from the time they are wee little - none of mine cared when I finally saddled them
I put saddles on them "sack them out" teach them that tarps and mail boxes are not monsters
I do teach mine to lunge around 2 years of age - just so they know in case they go lame and I need to lunge them to see their paces - or for the vet - I dont do it often or long - 5 minutes at a time - no cantering - just walk trot....
I bit them when the time seems right - and I start riding them when the time seems right
Rose I broke at 4 years of age - Dan I ended up breaking when he was 3 I think - I actually wanted to wait another year - but he REALLY needed a job. his rides were short - he never cantered until he was ummm 7? he was not as matured as I would have liked - but he was being horrid and NEEDED a job.
at 2 I think a horse should be able to
---tie
---lead
---have all 4 feet handled, trimmed, and have "pretend shoes" "nailed on" (gently banging on a horse shoe placed against their foot - gets them used to the sound and feel if they ever need to be shod - I think it makes them nicer horses in the end - esp if one ends up selling (and you cant guarantee that you will NEVER sell them) I also think it makes their first shoeing a nicer experience for them too.
--- have cloths, bags, saddle pads etc placed on their backs, heads whatever and accept it - even think its funny (my Rose had a sense of humor)
--- possibly - if the time seems right accept a saddle and being led with it on
--- Heart girth hugs
--- being touched EVERYWHERE
--- LIGHTLY (emphasize
LIGHTLY) lunge - as explained above to evaluate gait and check for unsoundness
--- Go over tarps, bridges, poles at walk and trot in hand
--- Take walks with a human - experience scary things and NOT run human over or jump in lap
--- take a bit if they seem ready (getting treats or grain once it is in - then take out)
--- go through puddles with out jumping or rushing through - streams would also be a plus!
--- give to halter pressure
--- Load in a trailer
--- lead, square up if possible
--- clip
--- not kill barking dogs - be afraid of cattle, goats, sheep whatever is near enough for me to lead them past
Personally I dont think you can "over handle" a youngster - I think you can over WORK a youngster or SPOIL a youngster - but other than things that give concussion or weight etc - I dont think you can over handle them - keep the sessions short and sweet -pay attention to their "brain muscle"
I introduce backing and more concussive work - when they LOOK ready - when they arent so awkward or gangly - and look more solid - like a horse instead of a youngster But I keep it light, varied and not repetitive or stressful on joints for quite a while