We know for a fact that she was 8 when she left Ohara and started to join different groups up to the current date.
We know Robin made an agreement with Crocodile 4 years before Alabasta arc about the polynegraphs
http://www.onemanga.com/One_Piece/203/07/
We also know Robin joined Crocodile's group at the age of 16 as per this image
http://www.onemanga.com/One_Piece/398/07/
Then again we hear from Spandam and was making fun of her last 20 years after Ohara; which implies that she is 28 years old.
I know the general consensus is that she is 28 but is it referenced anywhere in the manga specifically?