I bought a horse once who was only 4. He was the most sensible, sane horse I've ever had. I rode him for a couple of months and then turned him away to mature a bit. When I brought him back into work, I took the tack to the field, tacked him up in the pouring rain and rode him back to the yard. He didn't bat an eyelid. One day I decided to break him to harness and because he was who he was, didn't bother with the preliminaries - just attached him to the cart (I'm not recommending this by the way - I was young and didn't know any better). He did wonder for a few minutes what was behind him but I told him it was OK and off we went. Just in a straight line, up the lane but again, he accepted it all in great good humour. We actually used to use him as a cover horse when we were riding just-backed babies out for the first time. He was just born bombproof.

Same with another horse, again a four year old. Had it in to break and sell on. Stunning Arab-y skewbald. If you didn't know better, you'd have thought he was 12. Just took everything in his stride.

IME, if they're sensible at 4 they tend to stay sensible; they might go through a slightly livelier stage at some point but still have that nice, reassuring core of sensible-ness