Shelf Life?

Is there a shelf life on beer?? Is it longer or shorter if it is bottled or kegged?

Depends on the type of beer and the temperature of storage more than what container it is stored.

I've heard of people trying 15 year old beers that tasted fine.

Depends. What's the shelf made of and how many beers do you have on it?

As a general rule, higher ABV = longer shelf life. Higher IBUs also help perserve a beer. Like wine, cellering at lower temps also slows down the chemical reactions that cause both maturity and deterioration in a beer.

By style, it runs the gammet from Hefewiezens and light Wheats and Pilsners, which peak around 4-6 weeks after brewday to Barelywines, Belgian Quads, and some Imperial styles, which can peak 1-3 years after brewday.