Just have them safety inspected. The five year thing is a myth. You'll have people on here who will tell stories of tire blow outs and wrecks but the vast majority of those blow outs are caused by improper inflation and not age. Now there is an age limit, but it's not five years and it depends on if the tires have been out in the sun or not. I'd have no problem driving on 10 year old tires as long as they aren't cracked and look good. And at the very least if I was changing just based on age, I'd replace the front ones and not the back.