Age is a poor diagnosis
A brown, brittle paperback beside a much older flexible volume is not a contradiction. The two objects may have begun with very different materials. A publication date tells us how long a book has existed; it does not tell us how its paper was made or where it spent those years.
The Library of Congress explains that cellulose-chain length contributes to paper strength. Acids, in the presence of moisture, can shorten those chains. Many older Western rag papers retained long fibres, while later manufacturing methods produced different fibre lengths and chemical conditions. The broad story is a change in materials and processes, rather than a simple rule that old paper is better.
Sources: s1
Read the label carefully
The Library's preservation guide also distinguishes acid-free material from buffered material. A neutral starting pH does not by itself establish future stability. An alkaline reserve is relevant because acids can develop later. Lignin and storage conditions also matter; one reassuring word on a packet cannot describe the whole preservation problem.
For a collection survey, record visible condition separately from presumed cause. 'Pages break when turned' is an observation. 'The paper is acidic' is a diagnosis requiring supporting evidence. Keeping those statements separate makes a condition record more useful to a conservator and prevents a guessed mechanism from becoming inherited fact.
Decide what needs to survive
The preservation goal also needs a name: readable information, a binding, annotations or the complete physical object. Scanning a page can preserve access to its words. Conserving the book addresses needs that a successful digital copy may leave unresolved. The distinction helps frame a treatment discussion; it does not diagnose the condition of a particular volume.
Sources: s2