Mike, that does seem a very low weight for a female of that age but life long underfeeding could, I suppose, result in that. We took in a snake for adoption a few years back that was confirmed at 18 months old and a female. She was at that time the same size as a female of about 6 months of age. Not skinny, just very undersized in both length and weight. She'd been chronically underfed (they followed VERY poor pet store advice) so was just basically maintained without enough nutrients to grow much on.

We gradually upped her total prey volume and she went into the most amazing growth spurts you've ever seen. She's actually just laid her first clutch for us and is a lovely adult now and well within normal length and weight ranges for a female of her age.

It wouldn't hurt though to do a fecal on this rescue of yours. Poor care that resulted in her growing so little might also be affected if she's carrying a parasite load so probably best to rule that out from the get go.