His gravestone has his birthdate as Feb 15, 1860, but the 1860 census, taken in July with ages supposed to be as of June 1, listed an unnamed male infant, age 1. So either the birthdate on the gravestone is wrong and he was really born in 1859, or he was only 3 months old in June 1860 (4 months old in July.)

Since he was listed in the 1870 census as age 11, and in the 1880 census as age 21, the former is more likely. But leaving a child unnamed for over a year is also unusual, so it could be that his age was consistently reported high in the censuses.