Let's strive for accuracy, guys:
(1) He was 18.
(2) Based upon his physical confrontation with the clerk as he attempted to exit the store, it was, almost without question, robbery under Missouri law.
(3) Under Missouri's statutes, there is no separate statute for "strong arm robbery." That's a colloquial term, maybe even a term used by the police, but it is indistinguishable from "robbery." Strong arm robbery is an inflammatory term and, I would suggest, is best to be avoided. Missouri has robbery 2 (the forcible stealing of property) and robbery 1 (robbery 2 + the use or threatened use of a dangerous weapon or the causation of serious harm to the victim). This was robbery 2, unless you have evidence of the additional elements of robbery 1?
(4) "Roughing up the old man" (i.e., assault/battery) is what elevated this from theft to robbery. So it's inaccurate to say that it was robbery + assault.