If a furnace or water heater is built before about 1980, the ASNI standard does not have a date (if it has a date, then it was built about 1 - 2 years after the date).
I had an idea yesterday to write down the pre-1980 ansi standard ID of z21.49.
I did a google type search for “ansi z21.49” and lo and behold (eureka) on standards.gov they listed it as a 1975 standard. Estimated date of install = 1977. Age is about 32 years (I had estimated it at 30+ years)