Many young men joined whatever unit was handy at the time. My 17 year old GGF of Morgan Co, Mo joined Co K, 25th Illinois Infantry while they were encamped at Otterville, central Mo. in Sept 1861. A history of the 11th Mo indicates they were mustered at St Louis and attached to Cairo, Illinois district. Try this URL:
http://searches2.rootsweb.com/cgi-bin/ifetch2?/u1/textindices/M/MO-CW+2001+8301693664+F