I sincerely apologize--I seem to be having this problem with all the ladies in my life lately.
As for your question, JETPILOT pretty much hit the nail on the head, but I'd like to add a few things. While maintenance guidelines are the same for a particular commercial aircraft, that doesn't mean an airline can't exceed them if they desire. Additionally, some airlines with less cash flow may cut corners and it may eventually lead the carrier into problems with the FAA.
As already mentioned, the theoretical life of an aircraft is indefinite; however, airliners get a lot of abuse, regulations change, and more efficient equipment becomes available. These factors combined limit the "useful" life of an aircraft. There are many 30-year old "Frankencrafts" serving with U.S. airlines today--particularly with Northwest. Eventually, these will be sold to secondary markets, perhaps converted to freighters, and they'll serve significantly longer. In the end it comes down to money.
I'd also like to point out that an aircraft's age is determined more by its flight hours and number of cycles than years. Aloha operated a 737-200 that was by no means the oldest aircraft flying, but the high-cycle environment, frequent pressurization, and the salt air eventually caused the fuselage to fail spectacularly in flight. This is a good example of how planes become more expensive to maintain as they get older; Aloha must now be very diligent in inspecting for cracks on their older aircraft.
Another example: The B-52 was introduced around 1954. It's still serving the U.S. today, and it has been announced that they'll will continue to be utilized until around 2037. The "newer" B-52s are only about 38 years old, but that still puts them around 75 years old when they are retired! Most B-52 crews already are younger than the airplane they fly. It amazes me that such an old design can still be viable. Then again, the government doesn't have the same financial constraints as the airlines.
I saw a vintage WWII B-17 recently here in my neck of the woods. It was a beautiful airplane! It costs them an arm and a leg to maintain those things, but it's worth it; they are flying history and the sound of those engines is unlike anything we're familiar with today.
I am by no means an expert on the subject, but I hope that gives you something else to think about.