What does the phrase "are of full age... and by reason of use have their senses exercised to discern" mean in
Hebrews 5:14 “But strong meat belongeth to them that are of full age, even those who by reason of use have their senses exercised to discern both good and evil.”
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What are those senses and how does one use them to get exercised? How does this activity look like in practical living?
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.
[Hebrews 5:14] simply references the age of "Mature Discernment". - Previously expressed about Immanuel's literary diet in [Isaiah 7:15] "that he may know to reject bad and choose good. " ( לְדַעְתּ֛וֹ מָא֥וֹס בָּרָ֖ע וּבָח֥וֹר בַּטּֽוֹב)
The senses are a metaphor for our muchness (heart, soul and mind) as addressed in Matthew 22:37
And He said to him, “‘You shall love the Lord your God with all your heart, and with all your soul, and with all your mind.’
From this point of view, basically you're asking "How to love God with all our heart, soul and mind?". That was asked in Christianity and was closed as opinion based.
One thing relevant to notice though is that no matter how we describe and exercise each sense to fully match the used words "all our", this command shows something humanly impossible and therefore our need for a Saviour.
