A possible answer based on this Gita verse is given below.
One's own duty, even if without excellence, is more meritorious
spiritually than the apparently well-performed duty of another. For,
no sin is incurred by one doing works ordained according to one's
This verse was interpreted by old commentators as jobs ascribed to ...
I think you meant in what stage of life should one shift focus to Moksha from Artha,Kama etc?
And that is possibly what was meant in the book you are referring to.My answer is based on this assumption.
In the Sadaachaaraa Varnana(Descriptions of righteous living) chapter of the Brahma Purana,Maharshi Veda Vyasa precisely answers the question you raised:
What every little I have learnt from great scholars I am sharing here.
As " D " mentioned -out of four "Purushardhas" Dharma is the pivot and one should adopt "Dharmic Kama" and "Dharmic Artha" and the fourth one will automatically come to you that is " Moksha".
you take a long stick - measure it into "1/3 of it's length" and start cutting ...