Seven to eight is a good age range to start with, and certainly older works as long as there are not soundness issues. HOWEVER - many other things to consider. Some friends of ours just bought their very first riding horses, one eight one thirteen. Sold as broke nice trail horses - I went out and rode them both, no particular problems other than one a little busy with her head. Fast forward a few weeks......the gelding has now bucked off his new owner and the mare is hard to control. They started tracking previous owners and found a great deal of history. The gelding had been abused and although he's a nice ride, some things set him off and he becomes dangerous. They are getting professional help with both horses now, and hopefully will have something to ride eventually but they will have a lot more than their original investment into them. Just goes to show, age is not a guarantee. Find out all you can about a horses history, know the sellers reputation, get a vet check.
Jan