Do you mean lifer for fifer? Even if he was a Lifer the age of six was to young to enlist. All men starting at age 18 of a community back then were to be part of an unorganized state militia so I doubt anyone younger than 16 (with parental permission) could enlist in the organized military service. What is your 3rd great uncle's name and where did he live? Have you found him in the 1850 census or any later censuses? Is he buried in a Veterans Cemetery?