The obligation to pay contributions ends at the age of 64 (women) or 65 (men). If the employment continues after reaching retirement age, the obligation to pay contributions, both for the employer and for the employee, remains until the end of the employment.

However, in this case, an old-age allowance (CHF 1400/month or CHF 16,800/year) can be deducted from the relevant income.

More details:

Leaflet 2.01 - Salary contributions to Old-Age and Survivors' Insurance (OASI), Disability Insurance (DI) and Income Compensation Insurance (IC)
Leaflet 2.05 - Payments received on termination of employment
Leaflet 2.06 - Domestic work