I'd be interested to see what the 1900 census says... this particular census year lists the month and year of birth for every individual. This is the only census so far that gives that much information about age, and it's super helpful, because as it's been noted, just giving an age leaves a lot of room for interpretation. Wish the other censuses had those columns.
April 1840 is what the 1900 census says. Which jives with the age given in the 1910 census assuming that the birth month is correct (which implies he was born in the last half of 1840, since the "as of" for the 1900 census was June 1, and the "as of" for the 1910 census was April 15.
Of course, if I found the right individual in the 1880 census, that individual gave an age of 30 to the census takers (implying birth year of 1849-1850).