I remember calling my mum's friends 'Mrs' until about age 10-12' which was usually about the time they told me to stop it.
My mum's neurotic BF wasn't the least bit interested im me (which is fine), but always wanted to be "Auntie" and used to get really peeved when I would say she wasn't my auntie because I was a bright wee thing precocious little shit
Weirdly I had a cousin 20yrs older than me and my aunt used to insist I called her "Auntie CousinsName" which I thought was ridiculous even then (and so did my mother).
I never it was odd for people to be Mr / Mrs / Dr and still don't but even as a kid I thought it was really off when adults tried to impose a relationship (auntie or granny / nanny whoever) that wasn't there.
IMO it's down to the adult to let people know how whether it's first name, Miss firetname or Mrs Lastname & kids should adhere. as long as you don't want to be everyone's bloody auntie but don't be surprised if people thing you have a pole up your arse if you still insist on Mrs when they're well into their teens 
Now don't get me started on cold callers who say 'Hi, is that Queen?' or worse, shop assistants who read your name off the card and say 'Have a nice day, Queen'. I have an easily shortened name IRL so I sometimes get that from randoms as well 