A chief constable has said that young men who have sex with girls aged 13 or over should not necessarily be classed as paedophiles.
Dyfed-Powys chief Terry Grange said he believed the word "paedophile" should only apply to older men who have sex with prepubescents.
He said it would not be possible to prosecute all boys who have sex with girls who are under 16 but over 12.
The charity Kidscape said the police chief was "on very dicey ground".
In England, Wales and Scotland, anyone who has sex with a boy or girl under the age of 16 has committed an offence. The age in Northern Ireland is 17.
If the child is 13 or older and consenting, the offence is classed as "unlawful sexual intercourse".
But if they are under 13 the offence is rape.
Mr Grange, chief constable of Dyfed-Powys police and spokesman for the Association of Chief Police Officers on child protection and managing sex offenders, said his views reflected this "grey area".
He told the BBC the "grey area" related to situations where "the girl is 13, 14, 15 and the boy is 16, 17, 18, 19, possibly 20".
He said: "If you prosecute each and every time a boy has sex with a girl under 16 and above 12, then we'd be in the schools across Britain, and in the youth clubs across Britain pretty regularly because, since I've been alive, it's been pretty normal.
"It's what teenagers do."
He said the age of consent was "artificial" and varied from culture to culture.
"The 16 age thing is an artificial barrier that we have in Britain. They have different artificial barriers in other countries," he said.
But he said sex between a man over 20 and a girl under 13 was clearly rape and UK law reflected this.
"If the fellow concerned - usually a man - is in his 20s, 30s, 40s and 50s, then I believe you should prosecute them ruthlessly, and the law allows for that," he said.
In a separate, earlier interview with the Sunday Times, Mr Grange was asked how he would define paedophilia.
His exemptions from the definition ranged from teenage boys to men in their 30s, which differed from his later interview with the BBC in which he said the upper limit was 20.
He said: "We are talking about very young people [when] it is paedophilia, and I wouldn't want to mix up kids who are just the wrong side of 16 with paedophiles.
"I don't actually personally adhere to the 15-year-old being with a 20-year-old boyfriend being paedophilia, or even if the boyfriend is 30."
Official government guidance on prosecuting suggests that teenagers who have sex when one of them is below 16 should not necessary be prosecuted.
Mr Grange, who stressed his views were personal, said: "You take a look at the circumstances and try to make the right decision for that case.
"It may be nothing, it may be formal warnings, it may be prosecution."
Michelle Elliott, the director of charity Kidscape, which campaigns to keep children safe from harm or abuse, told BBC Five Live that using puberty as a guide to sexual maturity was difficult.
"Girls, nine, 10 and 11 are now post-pubescent. They're wearing bras, they're having their periods. They are no longer children in his definition.
"So I think he's on very dicey ground here. I think he was probably trying to bring up the teenage issue, and maybe got himself in deeper than he meant."