What is the ruling on the oath I took at the police station, and is repentance obligatory upon me, knowing that I was sixteen years old at the time and that ten years have passed since the incident?

Pure lying, without coercion or necessity, is one of the major sins. Its sinfulness and severity of prohibition increase when it is sworn upon, and such an oath is a Ghamus oath, which is one of the major sins. Sincere repentance for this is obligatory, involving remorse and a firm resolve not to return to it. If there was an oath to speak the truth before the lie, then along with repentance, an expiation for breaking an oath is required.