Get it looked at somewhere, are you able to get it to a repair shop? 15 yrs really isn't all that old if you've been taking care of it. This summer my Toro rider just stopped running and blew piles of white smoke into the air I automatically figured I seized the engine. When I called a local shop (Mass.) they said it sounded like a head gasket failure. I got it to them and come to find out the carb simply clogged and gas backed up into the oil. A good old fashioned tune up and carb cleaning fixed the problem and it's now running like a clock. Set me back a few hundred bucks but for now it beat spending 1,500+ for a new machine. Never know could be "nothing."
