"Gnawing the old" is when young people stay at home, and rely on their parents to support themselves. "Full-time children" take care of their parents and do housework as their "profession" It is like a "full-time wife", but it is also a contribution to the family and saves the cost of hiring a nanny.
It is conceivable that with the advent of an aging society, more young people may choose to be "full time children"