I'm nearly 40 ffs!
I just started a new job and my boss has introduced me by email to a supplier as "the new girl".
My lodger said "I bet you wouldn't mind if a woman had said it", but that's not it at all. I wouldn't actually mind (much) if a colleague called me it, but my line manager or anyone senior? 
It's patronising, isn't it? And I'd bet good money that a 40 year old bloke would something like "the new guy", definitely not a "boy".
Grrr.