"I am seventeen years old."
According to my japanese mom, no one says じゅうしちさい everyone says じゅうななさい so I guess duolingos in the wrong here, but thats for age, the first is used for other things
From my understanding of it, なな is said any time the number in question is above 10 (17, 27, etc.), and しち is used whenever you want to say 7 by itself.
So 七 would be しち, and 十七 would be じゆうなな. I think there's similar rules for the other numbers as well!
Actually nana is used almost every time, only in some situations shichi is used, but in general no one says shichi, except when is needed.
Probably なな and よん are used when you do not want to attract bad luck (しち and し sound similar as しぬ = to die).
yes, that is the reason to avoid shichi, but there are situations that you have to use it.
when I was in Japan I heard if you knock on someones door 4 times (knock knock knock knock) they won't answer because "shi"
Duo is hit or miss with accepting kanji in answers (even though it's provided as a pop-up translation). Reported.