I purchased my 4 runner used it is a 2006 sr5 with 21k miles with the original tires on it. They still have a lot of thread but I noticed on outer side of tire I noticed many small cracks by the outer thread, is this safe ? should I get new tires? The previous driver rarely drove vehicle , it was parked in an indoor garage most of the time. should I be concerned that the tires are 5 yrs. old?