How old do dolphins get?

The maximum age for bottlenose dolphins is between 40 and 50 years. The average age a dolphin can get (the life expectancy) can be calculated from the Annual Survival Rate (the percentage of animals alive at a certain point, that is still alive one year later). For the dolphin population in Sarasota Bay, the ASR has been measured to be about 0.961. This yields a life expectancy of about 25 years. For the population in the Indian/Banana River area, the ASR is between 0.908 and 0.931. This yields a life expectance between 10.3 and 14 years. So the actual life expectancy differs per region.

Spotted dolphins don't do well in captivity and  because a wild study hasn't been done long enough to tell exactly  how long they live we have to guess based on what we do know so far.   Generally speaking, the larger the dolphin species, the longer it  lives.  The orca whale is the largest dolphin  yes it's a dolphin!) and can live for 80-90 years. Keep in mind that these are estimates and are based on ideal conditions.

R.S. Wells & M.D. Scott (1990)
Estimating bottlenose dolphin population parameters from individual identification and capture-release techniques.
Report International Whaling Commission (Special Issue 12): 407-415
S.L.Hersch, D.K.Odell & E.D.Asper (1990)
Bottlenose dolphin mortality patterns in the Indian/Banana River System of Florida
in S. Leatherwood & R.R. Reeves:
The Bottlenose Dolphin, pp. 155-164,
Academic Press
See also:

A survival guide to survival rates (online paper).