I'm in North London and feel younger than average with two children (aged 5 and 1) at age 35.
I'd say an older mum where I live would be someone having their first child over the age of 40.
I was talking to one of my friends from where I grew up (Norfolk) about how tired I am and she said how hard it must be to have a toddler at my age 
So what's average where you live?