In a crime scene, who would you believe?

Neither.

@Waluigi that is not an option, as you can see

Well that depends on the crime...and what you mean by troublemaking...if the kid is just obnoxious vs if the kid has a history of crime...If its theft and the teenager has a history of theft I'd probably suspect him, but if it's murder I would have no idea

definitely not the incorrigible one

The old lady. If the teenager was a " trouble maker ", it's most likely that the teenager would do it.

I didn't give too much details for a purpose

You are overthinking it. Tis not much thinking involved

Old timey lady