Draft their son in 18 years, I guess.
While the draft did drive plenty of 18 year olds to "voluntarily" enlist as to avoid service in the infantry, it did not take 18 years olds . Estimates are as high as 65% of all personal were driven by the draft in one form or the other, so I don't want to diminished that. It was so important that the DOD had serious concerns about the All Volunteer Force when it rolled out.
But 19 year olds were screened and selected by their local draft board for induction during the year they turned 20. Draft boards were assigned a quota to fill and would do so by selecting young men from their area.
The draft lottery was restored in 1969 to call men to service in 1970, and men born in between 1944 and 1950 were eligible. There after the draft lottery would select men who were to be 20 years old during the calendar year of their induction. For example my father was born in 1953, and his birthrate was included in the 1972 lottery for hypothetical induction in 1973.
Notice the year.... the lottery pulled from an initial pool of men as old as 25, and eliminated marriage and college as deferments. A lot of men who thought they had previously escaped eligibility now found themselves being drafted. That was a major turning point in resistance to the Vietnam War.