There's this UIAA document on ageing of ropes: https://www.theuiaa.org/documents/safety/About_Ageing_of_Climbing_Ropes.pdf
Ageing caused only by storage can almost be neglected compared with ageing during use.
These facts also hold for 10- or even 15-year-old ropes. This has been proved by many tests of such old ropes (not even 25-year-old ropes and one 30-year-old rope broke in tests...
A rope need only be discarded when the the sheath has been damaged such that the core is visible.
I’d be totally happy to use a 15 year old rope stored in a cool dry cupboard in my home. The BD study (one data point) more or less confirms this. I’ll try to find the link but IIRC Beal recommends a rope could be used for up to 10 years after sitting unused for 5 years. You can be sure they are being cautious. Storage in someone else’s garage might be a concern. Are you completely confident it’s been nowhere battery acid for example?
I'm currently using a old single rope at the climbing wall. I started using it after my short gym rope got too short to lower off with.It must be the best part of fifteen years old.I had a brief review of the reports linked to above and decided there seemed no evidence it needed retiring. It has not broken yet.