As long as the airgun is less than 500fps there are no regulations or rules on buying or selling written in a legislation as far as I know.
The 18yrs of age an individual stores "policy" not a law, rule or regulation....so as long as the store you are dealing with doesn't have an internal policy with an age restriction then you would be able to deal with them.
A pellet gun, nerf gun, paintball gun, airsoft gun, sling shot, etc. under 500fps has no "official" regulations.....only individual store policies dictate what they are comfortable with.
What I have written above is to the best of my knowledge.
LC
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