This news broke yesterday. The key sentence is
On his retirement Stirling said “This afternoon I scared myself and I have always said that if I felt I was not up to it or that I was getting in the way of fellow competitors, then I would retire”.
I have to admit that I felt a mixture of regret that an era has ended and sense of relief that he has decided to call it a day - on the track anyway. Hopefully he will still be around as motor racing's elder statesman for a while longer.