Hetty Feather is probably suitable for kids aged 10 or 11 and up. It's also more than suitable for teens and adults - good books don't require limits.
Her relationship with an early foster family, particularly foster brother Jem, provides a real heart (and real heart-break) for the book - as does the slow dissolve of many of her childhood dreams. It's not over-done, leaves some room for hope, and is really quite beautiful.
The writing is mature enough to please adults, but not too complex for a kid to understand. The prose is pretty damn good.
I personally didn't like the infancy parts - I know Hetty's supposed to be quite fanciful, but the first-person accounts of being a baby are just a little too unbelievable in my opinion.
Honestly, this was a fab book, highly entertaining, and certainly worthy of praise.