A lottery based on birthdays determines the order in which registered men are called up by Selective Service. The first to be called, in a sequence determined by the lottery, will be men whose 20th birthday falls during that year, followed, if needed, by those aged 21, 22, 23, 24 and 25. 18-year-olds and those turning 19 would probably not be drafted.

On this day in 1942, Congress approves lowering the draft age to 18 and raising the upper limit to age 37. In September 1940, Congress, by wide margins in ...
www.history.com/this-day-in-history.do?action=Article&id=63...

Conscription in the United States (also called compulsory military service or the draft) has been employed several times, usually during war but also during the nominal peace of the Cold War. The United States discontinued the draft in 1973, moving to an all-volunteer military force, thus there is currently no mandatory conscription.

However, the Selective Service System remains in place as a contingency; men between the ages of 18 and 25 are required to register so that a draft can be readily resumed. More recently registration is not required, instead information is collected for each person after their 18th birthday.
http://en.wikipedia.org/wiki/Conscription_in_the_United_Stat...

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Note added at 13 mins (2009-05-26 01:37:12 GMT)
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Serivço militar obrigatório