As far as I know, there is no age limit for coverage under ESI. When the salary of an existing member/employee exceeds the threshold during the contribution period, contributions are to be made based on the actual salary until the end of the contribution period. Only after that, the individual would cease to be a member due to the higher salary.
Of course, you can reemploy a retired employee, but why would you prefer to pay him in cash? How will you account for it? In such a situation, you can consider taking an insurance policy under the Employees Compensation Act, 1923, provided that his salary is more than Rs. 21,000 per month.