The two associations that represent security firms say they frown on clients that spell out age limits on guards in their contracts.
But they add that there is little they can do besides educating these clients not to reject older guards.
"As long as a person is able-bodied and passes the medical examination, there should not be an age limit for guards," said Mr T. Mogan, president of the Security Association of Singapore, which represents about 150 security firms.
Mr Robert Wiener, president of the Association of Certified Security Agencies, representing about 70 security firms, added: "If I see a contract like that, I will report it to the union."
He also encourages his association members to report examples of age discrimination to the authorities.
The fair employment watchdog, the Tripartite Alliance for Fair Employment Practices, said that one in five of the 303 complaints that it received last year was related to age discrimination.
Mr Nazarisham Mohd Isa, director of Jasa Investigation and Security Services, said that some clients prefer younger guards for their youthful image.
"We just have to try to persuade them that older guards are just as good," he said.