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
We also know Robin joined Crocodile's group at the age of 16 as per this image
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?