I lived in England for a few years in the late 90's. Just this morning I came across an old plastic film can with Teasel seeds I had collected in 1998. I was pretty sure they were dust by now but was amazed to see most of them intact! Out of curiosity I took one of them and cut it in two and was astounded to see white inside. Just for the heck of it, I've taken half and sown them in soil. It will be interesting to see if any germinate.

Being essentially weeds, I'm thinking they have a pretty long shelf life. Stay tuned for results!

It is good if gardeners test older seeds for germination.
I recently came across a few hull less oats seeds from 2011.
Ten out of ten germinated on moist paper towel.
But martagon lily seeds from 2011---only 2 germinated out of many seeds.

Let us know how the 15 year old seeds do?

I certainly will. I keep records of the last successful germination date of all my seeds and I also grow out 10 of the oldest seeds I have every year for fresh seeds. I've had amazing success with 10 year old datura metel seeds!

The Teasel seeds have remained remarkably fresh looking for their age. The digital age gardeners probably don't know about the plastic 35mm film canisters and how awesome they were for storing seeds.

I remember those!