I think it depends what you want to do with the horse. I just got a mare this summer who is about 25. I only got her cos she was going to go for meat, obviously we are not going to be going eventing or anything but I hope to bring her out for veteran showing next season. The youngest I have bought is my New Forest pony. She is 7 now but was only 8 months old when I got her.
My mare was rising 16 when i bought her i have had her 5 year in january and she as not aged a bit. The youngest i have bought is my nf he was about 5 and half months old and was supposed to be sold at three but looks like i,m keeping him now as i really like him.
when we looked for ours we were looking between 7 - 12 (wanted a horse that had been tere done that and got the tee shirt) but got a 5 year old jus because he was the right horse
At this stage, the youngest would probably be four, and the oldest probably about 15. I would only buy a horse older than 12 if it was a really fantastic been there done that competition horse, and I was in a position to keep it very long term.