My neurologist says that if a person didn't had history as a child ADHD isn't very likely. Is that always true?

Not always . History is not infallible. Some patients don't have the history of add explicitly in their past history, but from achievement or employment history a dr can assume they were add from other factors, such as report cards, academic performance, employment history. Also, medical conditions such as repeated head trauma or infection can predispose to add.
Not always. They probably meant that if a person didn't have symptoms of adhd as a child it isn't likely that they have adhd. You have to have them for the official diagnosis. However, sometimes symptoms are missed (especially add symptoms) until a person enters a more challenging environment. I know an er doc that didn't recognize her symptoms until she needed to care for several patients at once.
Not necessarily: Although adhd has classically been viewed as a disorder of children, more than half of the patients carry symptoms, or even the full adhd-diagnosis, into adulthood . Greater than 60% of the adult patients have at least 1 additional psychiatric diagnosis. Substance abuse disorders are seen in 10% of the patients. Heritability estimates range from 60% & is higher in adult form vs. Child adhd.