But you're the employment lawyer.

According to the General Regime of Social Security, workers (and their employers) are exempt from Social Security contributions if the worker is older than 65 and has paid contributions for at least 35 years.

And here's the meat that I was alluding to in a different diary:

Article 165. Incompatibilities.

1. The enjoyment of a retirement pension in its contributive mode, shall be incompatible with the pensioner's work, with the exceptions and in the terms  determined by law or regulation.

Added by the law 35/2002, of 12 July. That notwithstanding, people coming into retirement whall be able to make compatible the reception of a pension with a part-time job in the terms established by regulations. During this situation, the received pention will be reduced in inverse proportion to the reduction applicable to the working time of the pensioner in relation to that of a comparable full-time worker.

2. The discharge of a job placement in the public sector delimited by paragraph 2 of section 1 of article 1 of Law 53/1984, of 26 December, on Incompatibilities of the Personnel in the Service of the Public Administration, is incompatible with the reception of a retirement pension, in its contributive mode.

The reception of the mentioned pension shall be suspended for the time during which the said job is discharged, without increases [in the pension's value] being affected.

3. Reception of a retirement pension, in its contributive mode, shall also be incompatible with the discharging of [duties as one of the] Administration Officials referred to in article 1 of Law 25/1983, of 26 december, of Incompatibilities of Administration Officials (This law was repealed and replaced by Law 12/1995, of 11 May, of Incompatibilidades of Members of the Government and Administration Officials.)