The unmixed powders, such as XTOL or D-76, in their original sealed packaging can be expected to last several years.

However, with age does come increased risks of issues.

If the powder remains as fine grains and pours smoothly, then that is a good sign that there were no issues in storage.

If it is clumpy, has uneven texture or colours, then treat with very high suspicions.

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Powders you suspect are more than a few years old should likely be tested before use with rolls of film you deem important.

Extremely important rolls should probably always be processed in chemistry you've very recently tested with less important rolls. Film and chemistry is cheap, once in a lifetime shots aren't.

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• The older tins of developer are typically considered totally shelf stable if they're not damaged, and are potentially good for decades.
• The newer foil packets are apparently more prone to spoilage, and may quietly fail after only a few years with poor visual indication.
• Stock solution made from older powders can fail faster than normal due to being more oxidized than expected. In general it is good to avoid letting mixed stock solution sit too long, but aim to use up older chemistry sooner rather than later.

In general it is good to treat all chemistry with some suspicion and care, and increase your suspicions as it ages.