Here ist the official version from the CDC, Section Immigrant, Refugee, and Migrant Health.
https://www.cdc.gov/immigrantrefugeehealth/panel-physicians/syphilis.html
Scroll down to the 'Key Concepts' Box. There you can read "All applicants aged 18 years to those aged less than 45 years must be tested for evidence of syphilis.".
And the evidence of syphilis is provided by the blood test during the exam and is the reason for it. So old farts and teens are all set and the exam is a little bit cheaper ... 😁