No way i should think its the best thing because you dont want big skittis young horses that will spook at everything. You want something thats calmer a little slower than average and forward going. Just make sure you warm up properly you dont him getting stiff and getting artheritus be careful!

Goodluck :)

No not at all.....if he is happy to let her ride him then I wouldnt worry about his age.  I can imagine that he would be very proud to be ridden and as long as you just keep his age in mind and warm him up appropriately before you start working him.  Im a strong believer that as long as co-morbidities don't prevent them from being ridden and if they are generally fit and well we should encourage them to be lightly worked under saddle.  Like humans horses need to be exercised to remain fit and healthy and at his age leaving him standing in a field or stable will just cause a flare up of arthritus and he ll become stiff over time. My mare is 22 and she is still going strong she loves to be ridden and along as she is happy I will continue to ride her, I just make sure that I warm her up before we start I.e 10 mins loose rein on both reins, 

Good luck, and enjoy