I got marked wrong because I "missed" one of the solutions: Вам сім? , which as far as I know isn't an acceptable translation. Was I wrong?
Вам сім років should also be accepted. It's impossible to know if one should use formal/informal ви/ти without knowing the contents
It is accepted
Would you ever be formal to a seven year old? If he/she is not of royalty of course.
Maybe if you were a five year old?
Can't I say : ти сім років ? Is this wrong? thanks
Yes, this is wrong.