Wow, a 25-year old debt?? That sure sounds like a bottom-feeder to me! A bottom-feeder collector buys junk debt for often pennies on the dollar and does everything it can to get any kind of payment on it.
In most places, the SOL is about 7 years, but every state is different. Also, once you make any kind of payment on it, the clock is reset and the SOL starts again. I would send them a validation letter and ask them to prove that it's yours, for starters.