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Ma'Lik Richmond -- one of the high school football players convicted of rape in Steubenville, Ohio -- shouldn't be forced to register as a sex offender for the rest of his life because he's only 16 ... and therefor doesn't have a fully developed brain ... this according to Richmond's lawyer.

Walter Madison went on "Piers Morgan" last night and told the CNN host why he plans to appeal the case -- in which Richmond was sentenced to at least one year in juvenile prison for raping a 16-year-old girl with his fingers at a high school party.

As part of the punishment, Richmond will have to register as a sex offender for the rest of his life ... something Madison says is completely unfair.

"I don't believe that a person at 75 years old should have to explain for something they did at 16 when scientific evidence would support your brain isn't fully developed ... when evidence in the case would suggest that you were under the influence."

Translation -- if you're underage and might do something stupid, make sure you get drunk first ... so you have an excuse and can't be held completely responsible.

After the comment, Morgan immediately fired back with rational thought -- saying, "I got 3 teenage sons and when you get to 16, 17 ... your brain's developed enough to know you shouldn't be raping girls."