I don't have an exact answer as I have yet to see the legislative documents that created the Ga. State Guard, but would guess that your assumption is at least partly correct. I have studied some of the soldiers from one particular company of the State Guard from Walton County - Co. F, 9th Regt. Inf. "Joe Brown Guards" and hope this gives some insight.

Their first Captain, Dickerson H. Walker, was 40 years old in 1864. The second captain, Jett Thomas Mitchell (who had earlier resigned for disability from Co. H, 42nd Ga. Vol. Inf.) was 42 years old in 1864. Likewise, Sgt. George W. Malcom, Jr. was 18 years old in 1864 (He would later serve in Co. H, 4th Regt. Ga. Militia and be captured at Griswoldville).

I'll attempt to do some other comparisons to see if others fall within conscription age.