Andy, I had exactly the same happen to my BFG ATs at approximately 7 years. It gradually got worse and I removed the tyres at age 10 due to heavy cracking around the sidewall joint to the tread and also between the tread blocks to a lesser degree.
Towards the end I was concerned that the tread might delaminate from the carcuss, and was cautious not to run the car with full load at high speed, and also limited max speed.
The driving experience confirmed that the tyre rubber had gone hard, which will have affected the grip level, this wasn’t noticable but would have been a gradual degradation of grip levels. Replacement tyres transformed the ride, much more comfortable.
The set of BFG ATs prior to these lasted longer. In my opinion tyre rubber does not last as long as it used to before degrading. This isn’t a sole trait of BFG, I’ve experienced it on many different makes and types of tyres.
It’s a sign of the times and modern tyre manufacture in my opinion, more often the tyres will require changing due to age related degradation than the tread being worn too low.
In my experience anything over 6 years is good going for a modern tyre.