Prob under 7, but my horse is 9 and is still my baby! I think it depends when they wee backed and how much they have done as if they ahvent done much, they still need to be treated like they are babies as they dont understand what is being asked of them. If it is manners though, they stop being babies at 2!
I would say under 6 it can still be used as an excuse for babyish behaviour....LOL! But don't think a horse is fully mature until about 8 - check out the ages of international sj horses (never usually under 8) and dressage horses under 10 is young....
By 6 they should be old enough to know better (assuming they were started at 3 or 4).
There was one particular incident last summer when I was minding my own business waiting in a collecting ring on Lexi (who was 4 at the time), insisting she stood politely and watched the class. I got chatting to the girl next to me who was on a bad mannered thing that was napping, head all over the place, refusing to stand still etc. etc. (you know the type). She kept apologising for her horse's bad manners and saying it was 'just a youngster'. Anyway, it emerged her horse was 8 and she had owned it since it was 4!! I didn't tell her how old Lexi was.
I think that bad behaviour is bad behaviour and people generally use the excuse of a horse being young for far too long!
The term 'green' means something different to me - that is irrespective of age and relates to level of training at the job in hand (so a 10 year old advanced dressage horse could still be 'green' cross country).
I would say 6 or 7 at the oldest. It does really annoy me when people let there horse's off with bad manners or only doing minimus for years as there horse is only 5,6, young, 8! And they think their manners will get better with age with never telling their horse off as its only young?
Like sillymare said there is a difference between young and green!