Is it possible to "roll over" either a 401K or a 403B into a 529?
Any general rules on withdrawing money from 401K or 403B plans. When
do you have to withdraw monies, and when is the tax benefit greatest?

Age 66: I'm in California; I want a 529 for a grandson in Colorado.

Sorry KDW, no such luck with the rollover.  Go to:
http://www.savingforcollege.com/529_plans/ezines/guru.php?guru_id=11

A general rule is to enjoy the tax-deferred gains in your 401k as long
as possible.  However, you will need to start withdrawing from it by
April 1st of the year following the year you turn 70.5.  The
government changed the Required Minimum Distribution table that allows
you to withdraw less than before, but you still have to do it.

When looking for a 529 plan, make sure you pay attention to the fees. 
That savingforcollege.com website is pretty good.  I am sure your
grandson (and son/daughter!) will thank you for whatever help you
provide.