Count the defects, not the birthdays
Roof age is useful background but condition matters more. One broken tile on an otherwise stable roof is a different case from widespread slipping, decayed battens and brittle underlay.
Ask which defects are local and which are present across the whole slope.
Repeated repairs tell a story
Look at previous work. Numerous hooks, clips, patches, cracked mortar and mixed replacement pieces can show a roof has needed regular attention. That still does not prove renewal is due, but it helps judge likely future maintenance.
If the same type of failure keeps appearing in new places, the underlying fixing system may be deteriorating.
What becomes visible when covering is lifted
Battens, underlay and fixings are largely hidden until work begins. A local repair can sometimes reveal their condition. Widespread timber decay or very brittle underlay changes the calculation.
Do not assume every imperfection beneath an old roof requires stripping the whole slope. Ask how extensive it actually is.
Get both scopes described
Where repair and renewal are both reasonable options, have the contractor explain what each would include. A vague 'new roof' price is hard to compare.
For renewal, details such as underlay, battens, ventilation, leadwork, ridges, gutters and disposal should be stated. For repair, identify the exact areas being dealt with and what remains untouched.