It is supposed that, by that age the teacher has the skills development, the maturity and the knowledge of the English culture, along with a greater potential to adapt to the changes in an alien culture. There might also be some grounds to add that, perhaps the mature foreigner is perhaps assumed to be more morally inclined as well to reflect the moral principles of Chinese culture.

Hahaha, from meeting and knowing some guys in their 40s, i can tell you it's not going to work. I know guys in China in their 40s, no degree, teaching english, boozing, meeting the girls....