The victim suffered a serious head injury in the attack at an address on School Road in Heysham, on Thursday.
She was taken to hospital where she remains in a critical but stable condition, Lancashire Police said.
A 57-year-old man from Heysham, who was arrested on suspicion of attempted murder, is now in a secure unit.
A man who was arrested after an 85-year-old woman was assaulted with a paving stone has been detained under the Mental Health Act.