Hops are only harvested once a year per hemisphere. This requires that hops are packed and stored in conditions that optimise that just pelleted quality. The Northern harvest happens about now but we do not start receiving hops here to typically late March (apart from some urgent airfreight varieties). Clearly the suppliers want to move move most the previous years first and the simple fact is that well stored they as perfectly fresh as the incoming.
What is well packed and what is well stored and how long will hops last before they start to noticeably degrade, well clearly that depends on the packing and storage. Hops are not unusual in that the big killer is air, and specifically oxygen.
Most importantly then the hops need to avoid air, ideally this is vacuum packing, they need to be kept cold (freezing is not required but may not hurt) and away from light. The worst possible storage is in clear plastic bags, which have a high oxygen permeability, outside the fridge and in the light. Clear bags (such as oxygen barrier bags that have been vac sealed and kept in the dark) are fine, this very handy if you are repacking on household vac-sealer.
There is a measure, that goes back to the 70’s called Hop Storage Index (HSI). This the amount of alpha acid acid loss after 6 months at 25C. Not much use us really !!
Here is a link to a Czech paper on hop storage, they conclude “Each component of hops has its specific value in beer brewing. These experiments have shown that the only safe way to preserve bitter acids is to store the hop pellets in a chilled space without air access.” https://onlinelibrary.wiley.com/doi/full/10.1002/jib.40
Correctly packed and stored your expensive dried hops will have no problem maintaining their quality over three years and probably more. Badly packed and after 6 months things start going cheesey.