YouTube talent, however, may not have much control over which ads display alongside their content, with many managed by multi-channel networks.
Multi-channel networks act as middlemen between advertisers, YouTube and YouTube stars on endorsements, book deals and other commercial activities, taking a cut of the revenue.
An investigation by the Independent found 30-second pre-roll video ads for sugary sweets and drinks playing before some of Zoella’s content, which it claims is targeted at children aged between 11 and 17 years old.
Worryingly, it also found a gambling site ad in one instance. It found similar ads running against content by Alfie Deyes, another prominent vlogger.
But a source with knowledge of the matter has told Marketing that Zoella’s channel is controlled by a multi-channel network, StyleHaul, likely depriving her of direct control over which ad categories display by her content.
Who controls ads?
YouTube channel owners do have a fair amount of control over which ads show on their content, and can block either by category or by advertiser. For example, a channel owner could bar all advertisers in the ‘food and beverage’ category, or a specific brand.
Jamie Oliver's company, which has urged YouTube stars to be proactive about blocking ads after the revelations, has blocked specific advertisers by name, thus ensuring junk food ads don’t appear by his content, the source told Marketing. Oliver’s channel, however, is controlled by his own company.
Advertisers are not necessarily in control of when and where their ads are served
The source also shot down claims that Zoella’s content is actually seen by a young audience, citing analytics data that shows the "vast majority" is over 18 years old.
Marketing has not seen Zoella's YouTube analytics data, and the star herself has said publicly that her target audience is aged between 13 and 17 years old.
Writing in the Guardian in November, she claimed to have a "young audience". But in the Financial Times, she has claimed men make up about 9% of her viewers, most of whom are aged between 45 to 50, suggesting her subscriber base is broad, but the most vocal viewers are younger.
Ian Twinn, director of public affairs at ISBA, said: "Jamie Oliver raises a fair point. Vloggers’ own content is very much covered by the ad rules if they are paid or rewarded for mentioning a product.
"The trouble comes in how ads are served by providers like YouTube. Advertisers are not necessarily in control of when and where their ads are served."
Twinn noted TV ad rules were necessarily stricter because ads are "pushed" to the viewer. A viewer chooses to visit online properties, however, and search out specific content.
He added: "The context of the ad needs to be taken into account. Rules cannot be made up to stop children deliberately going online to access content that is not designed for them.
"There has to be a sense of proportion online. If content is aimed at young children then commercial messaging needs to respect the level of children’s understanding and respect concerns for their health."
Zoella's talent management company, Gleam, has not responded to a request for comment.