I'm not in London. I do live in the South but lived in the North West when we started having kids.
I had mine in my late 30s and early 40s. I don't recognise Billy's idea of being so old in the playground because both here and in the NW every other mother is about the same age. The youngest mum in my NCT group of 8 was 34.
I'm amazed that anyone can manage it in their 20s tbh. DH and I met at university but no way could we have afforded a baby at 25. I finished uni at 22 then debt and both trying to establish a professional career and buy a house and build up savings. That's before we even get to enjoying stuff like travel and debauchery! 
Becoming a mother has changed me completely and it is consuming. In glad I waiting until I'd done lots of other stuff and could afford to sah for a bit and had babies at the same time as all my friends.