The conventional way to measure a bull market is from its previous bottom which for this rally, it started from March 2009.
However, Morningstar defines expansion as starting only after the market reached its previous peak. Under such a definition, this bull is only 39 months old.
As for some reference points - the bull in the 1950's had a life of 146 months, in the 1980's had a life of 134 months, and in the 1990's had a life of 135 months.
In terms of returns achieved, in the 1950's, stocks soared 698%, in the 1980's, stocks returned 435%, in the 1990's, stocks rose 523%. By comparison, today's expansion only generated a return of 57%.