My dad has just done the same with his 54 plate Yaris. Original tyres had quite a bit of life left in them, but were 6 years old, hand book states that tyres over 6 years old should be checked by a mechanic and an assessment made. My dad only does 3000 miles a year, so he just had all four changed to Michellin Eco for £210 all in, which should last another 6 years by which time he'll probably not be driving anymore.

If the age of the rear tyres is the same as the car then they are 9 or 10 years old

Good advice to change tyres after they reach certain age, I was told about 10 years but I suppose that Toyota are being cautious by stating 6 years.

So best to change all 4 tyres.

I was told on front wheel drive if all tyres are same size, when buying new tyres put them on the back and move the rear tyres to the front. Doing this each time the fronts wear out will ensure that you don't have the tyres too long.

Happy motoring