I think it would be worth investigating. Take a saw out and see if you can get it loose. Then it comes to is the juice worth the squeeze. If you're in desperate need of a stave or two then go for it. Otherwise I would say leave it and go find a good living tree to harvest and dry properly. you'll exert the same amount of work.
As far as age goes, I've personally made bows from wood that was 20-30 years old. Old fence posts included in that. It used to be a thing in the south to cut down all the osage and use them for fence post as they are naturally rot resistant. There is a good chance that your downed tree is just fine to as long as it hasn't been wet for long.