I could be wrong but I'm not buying the stereotype that all young people fit into the categories of uninformed and gullible.
When you build logic on an assumption, as you did, then the conclusion obviously doesn't necessarily follow. We have no real evidence that advertisers think as you say.
The real story is not that young people are more gullible, but that older people tend to be more set in their ways, thus less likely to change their buying habits because of advertising. That's more the driving force here.