It's not how old you ARE, but how old you LOOK. Can you pull off at least 32?
(figuring she got engaged at the youngest around 18)
I played Adelaide when I was 20 (in a community theater production). I am 5'8", was about a size ten, and with a little aging make-up and blonde hair, pulled it off decently.
I think character/acting/voice is more important than if you look exactly the right age. costume and makeup can always help with that.
Break a leg!