Jamieson, Fausset and Brown on Hebrews 5:14

14. strong meat--"solid food."

them ... of full age--literally, "perfect": akin to "perfection" (He 6:1).

by reason of use--Greek, "habit."

senses--organs of sense.

exercised--similarly connected with "righteousness" in He 12:11.

to discern both good and evil--as a child no longer an infant (Is 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 He 13:9.