When should an asphalt shingle roof be replaced?
Assuming a shingle roof. When to replace:


It is leaking and can't be easily repaired.
It looks bad. You don't like the looks, or the city is threatening to fine you for code violations.
You want a better roof. Better hurricane resistance in my case.
An unbiased expert says it should be replaced.


The climate affects the expected life--heat is hard on shingle roofs. Are you comparing the lifespan to other shingle roofs in your area? That is a better indication than opinions from people around the world.