Heb 5:11-14 says (BSB):
We have much to say about this, but it is hard to explain, because you
are dull of hearing. Although by this time you ought to be teachers,
you need someone to reteach you the basic principles of God’s word.c
You need milk, not solid food! For everyone who lives on milk is still
an infant, inexperienced in the message of righteousness. 14 But
solid food is for the mature, who by constant use have trained their
senses to distinguish good from evil.
This fairly literal version makes the sense clear and puts it in context as well. The author of Hebrews is chiding his readers that their spiritual maturity is less than it should have been.
The metaphor here is that of food representing spiritual teaching: milk for babies represents simple teaching that is given to new Christians; while solid food is more advanced teaching for more mature Christians.
The KJV phrase "full of age" is unfortunate and an interpretive translation that does not exist in the Greek. It translates the word τελείων meaning (here) "mature" as in most versions.