Why would you use a formal form for someone who is likely 7 years old? :-P
The question could be addressed to a group of children and the you have to use "ви."
It's possible to use "Ви" form even with 7 year olds. Could be simply used out of respect.
Once I was seven years old.
Years and old were not included in the multi choice options so accepted Are you seven. If you had to type the translation it would have been marked wrong
Difference between тобі і вам?
Singular informal and singular formal or plural
Why is the Dative case used here and not the Genitive case?
No reason, that's just like that
It's like "(Are there) seven years (behind) you?" There is a sort of implied prepositionality linking the years with the pronoun as the possessor of those years. I hope that makes sense...