Is a 18 year old mare quarter horse too old for me?

Im 13 and my sister is 19 and we are looking to buy us a good riding horse that will last a long time and that can canter and trot with us. shes a quarter horse that is 15.3 hands she a little bit of arthritis but living longer (longegivity??) runs in her blood. so do you think this horse is too old or would work out fine?