Per the DSM-5: "Several inattentive or hyperactive-impulsive symptoms were present before age 12 years" is one of the diagnostic criteria.

Source: I'm a doctor, also https://www.cdc.gov/ncbddd/adhd/diagnosis.html

Given their source, I think either what they wrote is not what they meant, or they misread the DSM.

Specifically, it’s important to differentiate between symptoms “emerging” in adulthood vs. being identified in adulthood. What they wrote implies that most adult diagnoses are likely not ADHD if they did not get a diagnosis earlier in life.