More information on your daughters experience and the horses personality would help us give you more constructive answers. As for age, as long as he is a healthy horse, its just a number. Ive seen a 40year old arabian still doing trail rides. Grant it her work load was limited but she was a healthy and happy horse. When purchasing an older horse, always ask what feed and supplements they are being given (if any) and ask how they are in the winter. I find some, not all, aged horses tend to have joint issues when the weather is cooler. (I'm not sure where your located but i'm up in Canada lol). Also, if possible, have you vet come and check him out, just to make sure there are no serious issues.
A Dog looks up to a man,
A cat looks down on a man,
But a patient horse looks a man in the eye and sees him as an equal