The main problem is the use of "their" to mean "his or her". Because it's commonly used for plurals it seems to refer to the marketing campaigns.
I would suggest making influence plural. This would avoid the need for an article.
Then we could re-organize it a bit:
The materialistic longings of juveniles make them more vulnerable
to external influences such as (the numerous) marketing campaigns.
"numerous" is a bit weak. Leave it out maybe? Or say "the ever-present marketing campaigns", or "the ubiquitous marketing campaigns" perhaps?
You could keep "juvenile" singular and say:
The juvenile, being more vulnerable to such external influences as the
ever-present marketing campaigns, tends to be easily enticed due to
his/her materialistic longings.
but remember not to use "their".
Do you think "enticed" or "ensnared" might be better than "captured"?