Use 'credulous' to describe someone who can be easily misled because of excessive trust or lack of skepticism. Young children are often credulous, accepting fantastical stories as true.
Pro Tip 2/3
Not Just Naive
'Credulous' specifically implies a readiness to believe false or exaggerated information, not just inexperience. Despite his experience, he was credulous enough to fall for the get-rich-quick scheme.
Pro Tip 3/3
Negative Connotation
The word often implies a negative judgment, suggesting weakness or foolishness in character. The con artist preyed on the credulous, selling them worthless fake gems.