It is not biologically possible for someone to have an uncle who is younger than themselves in the traditional sense. An uncle is typically the brother of one's parent, and they would naturally be older than their nieces or nephews.
However, there are certain situations where people may refer to someone younger as their "uncle" as a term of endearment or respect, even though there is no actual familial relationship. For example, in some cultures or communities, it is common for younger individuals to refer to older friends or acquaintances as "uncle" or "aunt" as a sign of respect or as a way to address someone with whom they have a close, mentor-like relationship.
So, while biologically impossible, the use of the term "uncle" to describe someone younger can occur in certain social or cultural contexts, but it doesn't indicate a biological relationship.