Buy social security, but the date of birth on the file is different from that on the ID card. Which should be the standard?

The stipulated retirement time is based on the earliest date of birth recorded in the files of the parties concerned.

Notice on stopping and correcting the problem of early retirement of enterprise employees in violation of state regulations (No.8 issued by the Ministry of Labor and Social Affairs [1999])

..... The identification of the birth time of employees shall be based on the combination of resident identity cards and employee files. If my ID card is inconsistent with the date of birth recorded in the file, the first date of birth recorded in my file shall prevail. It is necessary to strengthen the management of resident identity cards and employee files, and it is strictly forbidden to arbitrarily change the birth time of employees and fabricate files.

Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.