Seven and ten both fits here. This is a mistake in the exercise
The form exercise for this sentence had two correct answers, but only one was accepted. It is fixed now.
So if someone asked a kid how old he was and he responded, "Десять" would that be acceptable?
Yes, that's the usual way
I thought ten was дес'ять, not десять.
Same here so I chose один!
I just assumed an advanced one year old beer drinking honey eating vegetarian duck from Amsterdam living in Mexico.