One could expect a greater ad influence on younger children because they are less ad 'literate'; they are used to advertising. However, Livingstone & Helsper found that this is not the case. They found that advertising has a greater effect for 7-16 year olds than on 2-6 year olds.
Source: S. Livingstone, E.J. Helsper. Does advertising literacy mediate the effects of advertising on children? A critical examination of two linked research literatures in relation to obesity and food choice. Journal of communication, nr. 3 2006, p. 560-584.