Mandamumu, I agree that we have to be careful about statistics (as I've already said). The 75% statistic you mentioned seems to have come from Benson & Matthews 1995 study. The Home Office refer to this research in their 2004 report, "Paying the Price":
"[M]any men and women were first drawn into prostitution when they were children. In this country around half began their involvement in prostitution before their 18th birthday. Some studies suggest that the figure may be closer to 75%."
So clearly this isn't saying that the figure applies only to under 18s, and the figures used in that report are drawn from a range of research, not a single study. It's also worth noting that the report's focus is on street prostitution.
I do agree with you that we must approach the statistics that get bandied about with caution and follow these up wherever possible to put them into context.
Sorry about the length of this post, but here is the summary of the sources used by the Home Office, showing sample size and key findings on age of entry into prostitution (Annex C in the Home Office report):
Kinnell 1993
115 women in Birmingham
More than half became involved in prostitution before they were 17
Faugier and Cranfield 1994
250 women in Manchester
63% of women became involved in prostitution before they were 20
Benson and Matthews 1995
48 women across the UK
Three quarters were aged 17 or younger when they became involved in prostitution 40% were 15 or younger
Sharpe 1998
40 women in a ?northern UK city?
A quarter became involved in prostitution aged 16 or 17 8% were 15 or younger
May et al 1999
67 women working in three cities in the UK
Half (33) became involved in prostitution before they were 18
Campbell 2002
70 women in Merseyside
30% became involved before they were 18 Of those, 38% were under 16
Pearce and Roche 1997
46 women
27% became involved in prostitution aged between 12 and 15 years of age ? 75% before they were 20.
Pearce 2002
15 young women in a London Borough and a Northern City
9 became involved in prostitution before they were 16
Hester & Westmarland 2004
122 women in Hackney, Hull, Kirklees, Manchester and Stoke-on-Trent
76% became involved in prostitution aged 21 or younger. 52% were aged 18 or younger