Does an old roof automatically need replacing?
No. Roof condition is more important than age alone. A well-built older slate roof may still have significant life remaining, while a much younger roof with poor installation or widespread defects may need major work. Inspection should determine the decision.
Condition, not date of birth
Plenty of old roofs are performing perfectly well, and some much newer roofs are not. What decides the question is the condition of the covering, the underlay, the battens, the fixings and the details.
Anyone quoting for replacement without looking at those things is selling a product rather than assessing a roof.
How to get a straight answer
Ask for evidence — photographs of the areas of concern, and an explanation of what is failing and why. Ask specifically whether repair is possible and what its likely limitations are.
A second opinion is reasonable whenever replacement is recommended for a roof that has not been giving trouble.