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:
The Munis requested the Maharshi(Vyasa) to outline the rudiments of
‘Sadaachaara’ or the fundamentals of Ideal Values of Virtuous Life
viz. or the Worthy Principles of Moral Conduct / Good Behavior.
Grihastena Sadaakaarya maachaara pari-rakshanam,
Nahyaachaara viheenasya bhadramatra paratravaa,
Yagna daana tapaaseeha Purushasya na bhuyatey,
Bhavanti yah sadaachaaram samullanghya pravartatey/
Grihastaas or family-persons ought to observe and protect Sadaachaara
Pravartana as those without it would neither have ‘Iham’ / the
contentment of current life or ‘Param’of post-life; to those who
neglect the principles of Sadaachaara are not eligible to the fruits
of Yagna- Daana-Tapa
Grihastaas are expected to follow the basic principles of Dharma-Artha
and Kaama during the first three quarters of one’s life and in the
last quarter of life to activities pertaining to Moksha. Also, half of
one’s expected span of life is to be spent with the deeds oriented to
Nitya-Naimittika Karmas or daily and occasional tasks of virtue; the
persons concerned might perform such deeds as to involve
Bharana-Poshana or sustenance and ful-fillment of family needs. But
the last quarter of life should be exclusively spent on activitees
aiming at the collection of ‘Mula Dhana’ of fruits meant for the
aftermath of life . In other words, ‘Dharmaacharana’or practice of
Virtue has to be an under-current in the Samsaara Sagara in all the
phases of life, especially in the last quarter of life. Care must be
taken that each of the ‘Purusharthas’should not be contradictory to
each other. In other words, Dharma should propel such Artha that
should not inhibit Dharma; Kaama should not defeat the aspect of
Dharma and Artha and likewise Moksha has to be a logical conclusion of
the preceding Purusharthas:
So, assuming one lives for 100 years,he/she should exclusively yearn for Moksha and act towards achieving it from the 75th year onward.
Please also go through all the other important aspects which are highlighted in the passage above.