If your family tree includes anyone that served in the military from Pennsylvania, the Pennsylvania Digital Archives are worth a look. They have index cards available online for soldiers serving in the Revolutionary War through the PA National Guard in 1921.
I knew my great-uncle Julius King Councill had enlisted in WWI from Pennsylvania, but there was some additional information in these cards. I found a card for his service in the Mexican Border Campaign in 1916 and a record of his enlistment in the PA National Guard.
The card above shows that Julius was in the Company B of the 6th Infantry and had the rank of Private. It lists his occupation as Inspector and gives a physical description and age. Julius was born in September 1899, so he would have only been 16 at the time of his enlistment even though he claims to be 21.