CL, in fact, i can't find the regulation, although i've only seached in English. All that I've come up is this below and it does sure not prove the restriction is legal. Have a look at the wording below

2. What requirements foreign cultural and educational experts or foreign teachers should meet for working in China?

(1) The invited object shall be friendly to China and willing to cooperate with us. Not only should he be excellent in professional level that enables the working commission but be good in healthy condition.

(2) Where the invited object is a foreign expert, he shall obtain the teaching and research experience for 3-5 years or more. The long-term cultural and educational expert shall gain master or higher degree or titles above instructor level and corresponding experience, meanwhile, the short-term invited expert shall be in possession of doctor degree or titles above associate professor and remarkable achievement in such academic areas.

(3) Where the invited object is the ordinary foreign teacher of language, he shall have gained the bachelor or higher degree and professional training on language teaching as well as some amount of language teaching experience.

link
http://www.china-tesol.com/SAFEA_Guide/_1_SAFEA_2002/_1_safea_2002.html

Now, I don't think any local authority wishes to be as straightforward as we would like to see. Also, I don't think the translations are as accurate as they should be. We're dealing with not so "transparent" culture.

From my experience, there're foreign teachers that've been denied work permits due to their age and I have read the regulation on daves esl site where some have brought up the regulations about the age limits as well as about the 5 year expert limits. Unfortunately, I am unable to bring the topics/regulations/links up here. Maybe some that're still daves members could find out about it.

Cheers and beers