them ... of full age - literally, “perfect”: akin to “perfection” (Heb 6:1).
by reason of use - Greek, “habit.”
senses - organs of sense.
exercised - similarly connected with “righteousness” in Heb 12:11.
to discern both good and evil - as a child no longer an infant (Isa 7:16): so able to distinguish between sound and unsound doctrine. The mere child puts into its mouth things hurtful and things nutritious, without discrimination: but not so the adult. Paul again alludes to their tendency not to discriminate, but to be carried about by strange doctrines, in Heb 13:9.