Goldfish are a species of carp which can and will grow to be large sized fish.  They have a lifespan of 25 or more years if they are kept properly.  They are omnivorous, however they mainly live on algaes of different types.  These fish should not be keep in a bowl at any time in their life.  They say that the goldfish will only grow to the size of it's environment, I'm sure you have heard it.  This is "partially" true due to the fact that a smaller tank usually means less than perfect water conditions, as well as less food for them to eat.  Goldfish produce a lot of waste and are far from the perfect pet for a fish bowl.  They should be in systems of at least 50 gallons or more.  More is better, of course, because it's less work to keep the water condition pristine.  If you would like to get a fish that will live in a smaller system and is much easier to care for, check out a betta.  They are not only pretty, but don't produce much waste (monthly 50% water changes will do fine).   But remember, beta's should have at least one gallon of water volume.  If you would like to try goldfish get yourself a nice outdoor pond.  They are much easier to handle and the goldfish will do far better than in a small indoor tank.