Could i buy a gun if i were diangosed with a mental illness in my teen years?

Question

or be a cop?

Answer 1

that depends and the severity of the illness and if you have been deemed complete cured. You must take no medication and there must be no chance of you becoming sick again.

Answer 2

Your individual state may have other requirements, but Federal law [ BATFE form 4473, section 11 F]

has specific requirements. See the link.