NET
20 Brothers and sisters, do not be children in your thinking. Instead, be infants in evil, but in your thinking be mature.
Darby
20 Brethren, be not children in [your] minds, but in malice be babes; but in [your] minds be grown [men].
KJV
20 Brethren, be not children in understanding: howbeit in malice be ye children, but in understanding be men.