Rule 601 Competency to Testify in General

Almost anyone can testify as a witness in court. There's no automatic ban based on things like age, mental illness, or a criminal record. The law starts from the assumption that a person is allowed to take the stand and give testimony.

There are a few exceptions found elsewhere in the rules that can stop certain people from testifying about certain things, or that limit what they can say. But those are specific, narrow situations. Unless one of those exceptions applies, a witness is presumed capable of testifying, and any concerns about how believable or reliable they are get sorted out by the jury, not by keeping them off the stand entirely.