Seventies,
Analog tape is surprisingly robust for storage. It is usage that degrades it. The biggest concern is break down of the underlying binder for the magnetic materials and the plastic substrate. If really old it is acetate and very prone to mechanical failure. You can get some layer to layer bleedthrough on thin cheap tape. You don't lose high frequency from storage but you may add noise. The plastic in records can also slowly decay albeit slowly.
Analog tape is surprisingly robust for storage. It is usage that degrades it. The biggest concern is break down of the underlying binder for the magnetic materials and the plastic substrate. If really old it is acetate and very prone to mechanical failure. You can get some layer to layer bleedthrough on thin cheap tape. You don't lose high frequency from storage but you may add noise. The plastic in records can also slowly decay albeit slowly.