If the couple involved don't meet any of the three basic
requirements in the Marriage Law the marriage is deemed
invalid:
1. Both parties are willing to get married to each other.
2. Both parties are of legal marrying age: Male over 22 years
old, female over 20 years old.
3. It's a monogamous marriage, neither party is already
married.
(Shanghai.gov.cn October 17, 2006)