.....so when we reach maturity ie 40, then we have to do the things that a mature person would do as described in 46;16. Some of us may already be doing them before 40 but the demarcation line as far as God is concerned, is 40?
The verse reminds me of the difference between me and one of my brothers. It fits the verses perfectly. My parents do not approve of him because some of his character flaws. I have known this for a time and I know he does not really have faith. My parents it seems have lost hope in him. My mother refused to acknowledge his flaws and even once accused me of being jeolous cause his life was more successful than mine at least financially and he did not have the same challenges as I did. She now flipped 100 degrees on him and complains repeatedly about him. Not something easy to do for parents. He is stingy and selfish and never cares about religion. .I think the verse is talking about it from a parents point of view. Notice the previous verses and how it talks about it.
15. We have enjoined on man kindness to his parents
: In pain did his mother bear him, and in pain did she give him birth. The carrying of the (child) to his weaning is (a period of) thirty months. At length, when he reaches the age of full strength and attains forty years, he says, "O my Lord! Grant me that I may be grateful for Thy favour which Thou has bestowed upon me, and upon both my parents
, and that I may work righteousness such as Thou mayest approve; and be gracious to me in my issue. Truly have I turned to Thee and truly do I bow (to Thee) in Islam."
16. Such are they from whom We shall accept the best of their deeds and pass by their ill deeds: (They shall be) among the Companions of the Garden: a promise! of truth, which was made to them (in this life).
17. But (there is one) who says to his parents, "Fie on you! Do ye hold out the promise to me that I shall be raised up, even though generations have passed before me (without rising again)?" And they two seek Allah.s aid, (and rebuke the son): "Woe to thee! Have faith! for the promise of Allah is true." But he says, "This is nothing but tales of the ancients!"
18. Such are they against whom is proved the sentence among the previous generations of Jinns and men, that have passed away; for they will be (utterly) lost.
19. And to all are (assigned) degrees according to the deeds which they (have done), and in order that ((Allah)) may recompense their deeds, and no injustice be done to them.
Just shows to understand some verses of the Koran you need personal experiences to understand the meaning. But eitehr way you can usual tell a person but parents may be in denial till it becomes clear the child they had is going astray.