Given the well-documented rarity of modern centenarians [1][2], it's highly likely that Old Tom Parr did not live to be 150 years old - and is much more likely that his documented age is due to errors in recordkeeping.

[1] https://en.wikipedia.org/wiki/Centenarian

[2] http://www.nytimes.com/2010/08/15/world/asia/15japan.html