This summary is not the rule. The rule text controls.
Anyone can testify as a witness in court. There's no automatic ban based on things like age, mental condition, or a criminal record. The law starts from the assumption that people are allowed to give testimony.
There are some exceptions, though. Other rules or state laws might block certain testimony in specific situations. For example, rules about privileged communications (like between a lawyer and client) or specific statutes might stop someone from testifying about certain things, even though they're generally allowed to be a witness.
The main point is that being a witness is the normal rule, not the exception. Courts don't need a special reason to let someone testify. Instead, anyone trying to stop a person from testifying needs to point to a specific rule or law that applies to that situation.
