Men enlisted for 12 years. You could either do 6 years in uniform and 6 years in the Army Reserve; or all 12 years in uniform. Men in the AR rarely recorded the fact on BMDs or the census etc. They just recorded their day job. The 'discharge' took place after the 12 years, not when they transferred to the Army Reserve. Service before the age of 18 did not count. So most men were discharged between the age of 30 and 35.
If a man was overseas when his 6 years were up, the army could keep him for up to another year, before allowing him to transfer to the Army Reserve to complete the rest of his 12 years.
Ken