15 years old? I'd use it for fire starter. It shouldn't be very volatile at this point. I'd still treat it as it is though. Especially the first time you use it. Stay upwind and as far back as you can throw a match. If it's dead calm out, don't pour it on and let it sit for a period of time before lighting it. If there are still volatiles, they are heavier than air and can creep out of the pit and spread to where you are standing. But at 15 years old, it should be about the equivalent of kerosene at this point.
I used to work in the petroleum and environmental industry and tested gasoline and soil & water contaminated with gasoline. If not treated, gas ages at a fairly fast rate and loses it volatiles leaving the semi-volatiles which are similar in chemical structure to kerosine. Gasoline's higher carbon-containing compounds have 9-11 carbon molecules and kerosine's compounds have 12-15. Thus ends today's science lesson.
But always use caution when working with anything flammable.