The "average" where I live is minimal exposure to guns at all so a single afternoon with some coaching would put a person well above average.
Type of shooting will also make a difference. Hand gun for self-defense purposes, shooting at a target 15 feet away is different than shooting clay pigeons with a shotgun which is different than distance shots with a rifle which is different than biathlon which is different than hunting of any type. Hunting will require skills and knowledge above simply shooting well.
Is someone going to pick up shooting in their thirties and get onto an Olympic shooting team of any type? Highly unlikely but I wouldn't be shocked over an outlier or two.
Those going into military service are going to be overwhelmingly 18-22 years old. Having an attention span longer than 7 seconds and some physical fitness puts a person over average.