Editorialist: Society is obligated to bestow the privileges of adulthood upon its members once they are mature enough to accept the corresponding responsibilities.
Conclusion
There is no reason not to grant 17 year old citizens all the privileges of adulthood.
Evidence
Society is obligated to grant all the privileges of adulthood upon its members, once they are mature enough to accept the corresponding responsibilities.
(so this author is assuming that 17 year olds are mature enough to accept the responsibilities of adulthood?)
Science has established that physiological development is completed in most people by age 17. This maturing process has been completed by most 17 year olds.
Evaluate
Wait, wait, wait. According to the conditional rule in the first sentence, we need to establish that 17 year olds are .
And the author is just saying that most 17 year olds have completed the process of physiological development.
Those are miles apart.
Goal
The author is abusing two different sense of the word "mature enough".
"Mature enough" in the principle is referring to one's intellectual and moral capacities. Are you responsible, wise, smart enough to handle adulthood.
The "maturing process" the author refers to in the final sentence is just talking about going through puberty physically.
This is a famous flaw called Equivocation, when you use the same term or concept in two very different ways.