At 19 and on active duty in the Army (many, many years ago) I had a very high level clearance. Wasn’t uncommon at that age if your MOS required it. My investigation proceeded while I was in basic and AIT and by the time I started AIT, I believe I had an interim secret which was necessary to start my training at the school and by the time I reached permanent duty station a few months later, the ts was completed and a few more months and the I was ready to be read in for sci.
Me at 18, Cheyenne Mountain NORAD Complex, Dec 1976.