Being a bit young to hold positions of authority, maybe he padded his age a bit. This appears to be his 90 day first service, listing age as 23:

http://www.sos.mo.gov/TIF2PDFConsumer/DispPDF.aspx?fTiff=/archives/AdjutantGeneral/Civil_War/ServiceCards/s845/0190.tif&Fln=S77993.pdf

But many of these cards have no ages listed at all.