What happens after the life span of a dental crown?

I'm thinking of getting a dental crown to cover up some decay on two of my teeth. I'm only 21 years old. To my knowledge dental crowns can last from anything from 5-15 years. I'll be getting a porcelain crown if possible for the best cosmetic look possible. They may last longer in some cases. Will the tooth be extracted or can another dental crown be placed and have another 10-15 years. Summary: What typically happens after a crown has reached it's life span?