Police, firefighters, ambulance and coastguards attended Fort Hubberstone close to Gelliswick beach in Milford Haven just after 19:00 BST.
She was airlifted to Swansea's Morriston Hospital.
Her condition is serious but not life-threatening, the ambulance service confirmed.
A 13-year-old girl has been winched to safety after falling at a disused fort in Pembrokeshire.