Is it cost-effective for 68-year-old people to buy social security?

No, but you can make it up, usually at retirement age. However, you can apply to the local social security bureau for one-time payment, which is subject to the regulations of the local social security bureau.

At present, the legal retirement age in China is 60 for men and 55 for women.

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