Migrant workers 15 can enjoy basic medical insurance benefits after retirement in Beijing.

Yes, you can check Article 11 of the Beijing Basic Medical Insurance Regulations online, which clearly stipulates:

"After the implementation of these regulations, those who have participated in the work and paid the basic medical insurance premium for 25 years for men and 20 years for women shall go through the retirement formalities in accordance with state regulations and receive the basic pension or retirement fee on a monthly basis, and enjoy the basic medical insurance benefits for retirees, and no longer pay the basic medical insurance premium."

Although it is not stated in the regulations that 15 pension insurance must be paid, you cannot receive a monthly pension without 15 pension insurance. It is stipulated that people who can receive pensions are eligible for basic medical insurance for retirees, so the implied meaning is that you must pay more than 15 of the old-age insurance.

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