I agree it totally depends on the horse. I know someone with an eighteen yo who is lively as anything. When I rode with the uni club I used to ride a 21 yo who was a bucking bronco! Its the 'You are as old as you feel' line...
From a care POV I start to think about looking after them differently at 18ish if they've not shown signs of aging before then but like everyone's said they're all different, mags as acted like an old man (mentally anyway, physically only recently) since he was about 12!
Mine is 19 and still acts like he did as a 4yo (tried to buck me off on Sunday) although I always have been very careful with ground etc - he would class as an older horse but like people - some don't act it
it does depend on the horse but certainly I think horses go on longer now than they used to and insurers need to get up to date and not class a horse a veteran until 18 or possibly 20