If I am arrested for meeting an undercover police officer who I thought was underage, is this considered entrapment?

Possibly.  A person arrested as part of a sting operation for potential sexual interactions with a minor may have an entrapment defense if they can prove they otherwise would not have committed the crime but for the inducement by law enforcement.  To have a successful entrapment defense, the conversations between you and the undercover law enforcement officer will be very important.  For instance, the questions and answers regarding who brought up the age and potential sexual experience with the minor are key to utilizing an entrapment defense.