Stock tyres on Classic 350, changing by km or by age?

Saw a thread on Team-BHP where people were saying tyres should go by 35,000 to 40,000 km but also by 3 to 5 years since rubber hardens regardless of tread left. My stock tyres still look fine on tread but they are getting close to that age window. Thinking of just changing them on time instead of waiting for km number. Anyone here replaced tyres purely because of age and not wear? What did the old ones look like when you pulled them off, cracked or just hard.