Out of ten shops visited by the volunteer for Sandwell Trading Standards, nine sold them products containing nicotine, such as e-cigarettes and e-liquids.
Council Trading Standards officers are advising retailers to be vigilant and responsible when selling e-cigarettes and liquid nicotine, applying the same checks as they do to the sale of any other age-restricted products.
In response to the investigation's findings, cabinet member for public health and protection, Councillor Preet Gill, said: ''Clearly this is an unacceptable result. It's important that retailers have procedures in place to ensure they do not break the law.
"Research suggests that e-cigarettes could act as a gateway to smoking tobacco cigarettes and the long-term health effects of using an electronic cigarette are still unknown.''
Bob Charnley, Trading Standards and Licensing manager, said: "It's our job to protect children by ensuring that businesses are aware of the changes in law and understand their responsibility to ensure such products are not sold to minors.''
He added: ''We have written to all the offending businesses reminding them of their legal obligations and that a further failure may result in enforcement action such as a criminal prosecution.''
This is despite it having been illegal to sell nicotine-inhaling products to anyone under 18 from October 1, 2015.
Retailers caught flouting the law could face a fine of up to £2,500, as well as a criminal record.
It has not yet been revealed which shops were caught out in the sting in Sandwell.
The ASH (Action on Smoking and Health) charity, which seeks to cut tobacco use, said about 2.8m adults in Great Britain use e-cigarettes. That is up from 700,000 four years ago. Users include 1.4m smokers and 1.3m former smokers.
But only 15 per cent of the public surveyed that e-cigarettes are 'a lot less harmful' than smoking.
Anybody who is unclear about the law should contact Sandwell Trading Standards on 0121 569 6580.