20.
Why I say 20?
Because by the time you are 45, your kid will be old enough to
support you and you can retire. 
actually, you do make sense.
by 40, you may be facing midlife crisis, the crush years. you
have to fulfill your commitments to both parents and children, and
you are prime target for retrenchment unless you are a senior
member of your firm.
so i guess it will be good to be able to send your kids off to
uni before you end up in this high risk age, and fulfill your
obligation as a parent to provide your kids with a uni
education.
