FS2000 is the best airline simulator you can get easily. There is an airline sim that is supposed to be better but it costs a few 100$ and you have to order it from sweden or somewhere. There are also some sims that do a better job of small aircraft, but FS2k is best for airliners. When FS2k2 is out will full ATC etc it will be the best by far
Like the guy above said, you will need more than FS2000 to become a pilot, but FS is a very good start, especially for a 16-20 year old.