Final answer:
The correction for sentence 3 is to change 'Turning' to 'At' to maintain parallel structure with the other sentences, making it consistent with the introduction of each age.
Step-by-step explanation:
The change that needs to be made to sentence 3 is to change 'Turning' to 'At'. This revision would create parallel structure with sentences 1 and 2, providing consistency in the way each notable age is introduced. Thus, the sentence should read: At eighteen, a teenager can finally be looked upon as an adult.
Being considered an adult in a legal sense typically happens at the age of eighteen, which signifies gaining legal responsibility for oneself. This age is an important transition point and is seen as the commencement of adulthood, encompassing the stages of early, middle, and late adulthood. Following this milestone, the next significant age norm established by the government usually comes at sixty-five, marking the start of old age and eligibility for senior benefits.