I have about 100 Panasonic BD-R discs, never used and sealed.
How long do they generally last if unused?
Is there any difference in writing aged BD-R (and discs in general) or it’s better to use them as soon as possible?
I think the answer is more around the middle actually. Something that sits and rots will be useless but using them up as soon as possible is wasteful.
If you don’t use them up right away then don’t worry about it. If by 50 years later for an extreme example you never used them and they don’t work because they degraded in that time then you bought more than you ever needed.
Panasonic BD-R standard/consumer media is considered to be of highest quality/longevity. Unfortunately they are no longer produced, so you can consider your stock as “as fresh” as you can get.
I believe, that there is no difference in that aspect compared to other media - I occasionally burn 15+ yr old good quality DVD±R blanks and the burns are still very good, so they don’t seem to fare just because on not being burned.