Yes they can, i am copy pasting rules from css2019, the part (f) below is relevant to your query:

Two years relaxation in upper-age limit of a candidate shall be admissible in respect of the following categories, namely:–
(a) the candidates belonging to the scheduled caste and Buddhist community subject to production of a certificate, in support of their claim, as set out in the application form prescribed by the Commission from time to time and issued by Deputy Commissioner, District Magistrate or such other person by whatever name called and authorized in this behalf;
(b) the candidates belonging to the recognized Tribes who are permanent residents of the areas mentioned-below and whose families have been living in these areas, subject to production of a certificate, in support of their claim, as set out in the application form prescribed by the Commission from time to time and issued by Deputy Commissioner, District Magistrate or such other person by whatever name called and authorized in this behalf:–
(i) Baluchistan;
(ii) Tribal Areas of D.I.Khan and Peshawar Divisions (including former
Frontier States of Dir, Swat, Chitral and Amb);
(iii) former excluded (Baloch) area forming part of the Dera Ghazi Khan
and Rajanpur Districts;
(iv) former Tribal Areas of Mardan and Hazara Divisions; and
(v) Upper Tanawal Area of Hazara Division;
(c) the candidates who are permanent residents of Azad Kashmir Territory, subject to production of a certificate in support of their claim, as set out in the application form prescribed by the Commission from time to time and issued by the Division to which business of Azad Jammu and Kashmir stands allocated;
(d) the candidates who are permanent residents of Gilgit-Baltistan including Gilgit, Diamir, Baltistan, Ghizer and Guanche, subject to production of a certificate in support of their claim, as set out in the application form prescribed by the Commission from time to time and issued by the Deputy Commissioner, District Magistrate or such other person by whatever name called and authorized in this behalf;
(e) the special candidates, namely, physically impaired, hearing or speech impaired and visually impaired, subject to production of certificate of disability in support of their claim issued by an officer of the Federal Government or, as the case may be, Provincial Government duly authorized in this behalf;
(f) in-service Government servants, armed forces personnel, contract employees, including the Government servants of Azad Jammu and Kashmir, who possess a minimum of two years continuous Government service as on the cut-off date subject to production of departmental permission certificate from the competent authority as set out by the Commission from time to time. Employees of nationalized banks, State Bank of Pakistan, Water and Power Development Authority, regulatory authorities, autonomous bodies and other semi-autonomous bodies are not entitled to the age-relaxation under these rules:


These rules in detail can be viewed at
http://www.fpsc.gov.pk/sites/default...ed%20rules.pdf

Page 3 and 4 discuss age limit and relaxation etc.