My geckos tend to be big, so I can't say what 'normal' weight vs. age should be. As long as your gecko is eating well and has a good fat reserve in the tail, I wouldn't worry.
One of my geckos is smaller than the other, tho the same age. She's just a smaller gecko, nothing wrong with her at all. There isn't really an average per sey, but as long as she's eating normally, there wouldn't be a problem with her size.