Foals achieve around 90% of their adult height by twelve months, on average. Bear in mind that these grpwth charts have been based heavily on TB derived horses; natives, drafts and cobs tend to gain more height later while QH types often reach mature height slightly earlier, so it's not a hard and fast rule.
Weaning at 3.5 months is not ideal at all. Yes it can be done, and yes, in parts of Europe it's standard in some places to wean at 4 months, but that does not make it the best thing to do. Foals can suffer growth retardation if they're not carefully fed when weaned early and it can affect mature height.
By far the most serious implications however are the results of several independant studies that have shown without doubt that early weaning (before the age of 5.5 to 6 months old) significantly increases the risks of the development of gastric ulcers in foals, and of 'vices' like windsucking and cribbing in adult horses.
Remember that a foal left to wean in a feral herd would usually be weaned when his mother drove him away, any time between 8 and 12 months old. Often the youngster wouldn't be weaned until the birth of his mother's next foal was iminent. Now in domesticated situations that's neither neccessary nor truly desirable; but when you consider the enormous stress that weaning causes (whether visible to us or not) it is a shame that so many people are in such a hurry to wean so early.
One independant study of foals artificially weaned at 3 - 4 months old showed that 90% suffered gastric ulcers post weaning, as opposed to less than 10% of foals weaned at six months old. None of these foals would have been diagnosed or treated for ulcers under normal circumstances; the ulcers were noted as a result of the studies. I find that really worrying.
