It would have been better if she was 13 or14 it would have made little sense then.Regarding advay's appearance I think he has been asked to look like that and it goes with his character as advay is someone who hates his life,forgets things,doesn't recognize food etcc.
Even i feel it would have been better if Advay was 16-17 & Chandni 13-14. 8 means she was just a child. This part of the plot definitely makes me uncomfortable