His age is never revealed, but assuming he joined the army as 18 years old and served seemingly in 1944 and 1945. He’d be around 89 or 90 in 2016 when the documentary about Lincoln Clay are being filmed. He’s then 41 or 42 during 1968, he would then be born in 1926 or 1927.
2016 - 90 = 1926
1926 + 18 = 1944.
1968 - 1944 = 24.
18 + 24 = 42
Remember it’s only an assumption, but it kinda stands correct time wise .