The phrase “old enough to know better” implies that he is also mature enough to drink alcohol, drive a car, vote, join the miitary, live on his own, et al.
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Only when trying to let Josh off the hook.
For the rest of us, it describes what a person of any age should know better than to do.
Like at age 10, one is old enough to know better than to shoplift.
or at age 14, one is old enough to know better than to sexually molest your kid sister.