All the recommendations I've seen are to inspect regularly at seven years plus for cracking etc but ideally replace them, with a max lifespan of ten years.
Out second car was near 8 years on original tyres because it was my mum's originally and does stupid low k's. Lived most of its life in the garage. Tyres still looking great and tons of tread. However when one picked up a screw I decided to dump them given their age.
The tyre place agreed it was probably the right decision (sure, they made money from it).
