Questions About Insured Children

How long are children, well, children? 

Obama Care says they are children to the age of 26 and covered by their parents' insurance.  I have questions.

When my son marries, is his wife covered under my policy or is my son covered by her parents' policy?  When the grandchildren come along, which policy covers them?

When I go on Medicare, does it cover my 24-year-old son, his wife, and his children?