Natsumi is a ten year old girl. The magic nuts make her physically age 10 years. Therefore, after eating the nuts, she would physically look 20 years old. Which is not a teenager. So why is the manga called Instant TEEN?
Because "Instant Adult" doesn't flow off the tongue as smoothly, I guess...*shrug*
The Japanese title is Otona ni Nuts, a pun on "Otona ni naru", Japanese for 'to become an adult', otona being the word for adult. The translators just wanted something catchier.