My first ever was on my 23rd birthday. Then about a week before my 26th. And then from 27 onward regularly.
Back then it was cards in phone boxes and most of the women were offering dominatrix and fetish-type services, at least in the area I used to search (Marylebone, anywhere between Gloucester Place to the West, Tottenham Court Road to the east, Wigmore Street / Mortimer Street to the South, and often Gloucester Place north of Marylebone Road)
Prostitution was mostly women hanging out in the streets around Kings Cross or possibly in Soho brothels.