Stevie Mambor is on trial in the ACT Supreme Court accused of having sex with a 15-year-old in April last year, using her to produce child pornography and possessing child pornography. But his legal team has argued their client had no idea the girl was underage when the sex occurred.

The jury has heard the troubled victim was using drugs, living out of home and no stranger to the juvenile justice system.

''Looking at [the girl's evidence], there are no two ways about it that she is a troubled young person,'' Crown prosecutor Sarah McMurray said on Thursday.

''She is a troubled child, and you will no doubt hear [in the defence closing] that she tells lies, that she takes drugs, perhaps that she's a bit of a manipulator … but she is a child nevertheless.''

But Mambor's counsel, John Purnell, SC, said the girl was ''not only totally unreliable, but she's also an admitted liar''.

Mr Purnell said the jurors' task was not to be ''censorious'' about lonely older men and young girls who sold themselves in exchange for drugs.

''It is confronting to see this young woman living a lifestyle you might think more suitable to a movie than a court of law,'' he said.

''But these are the facts in this case.''

The girl admitted, to police and in court, seeking out Mambor when she was desperate for drugs, and telling him she was 18.

She says she told him her true age months before he was arrested on April 21 and yet he continued to have sex with her.

Mambor, however, said he made the discovery himself, just days before his arrest when he was going through her bag.

The accused man, entering the witness box, told the court he found a card with her date of birth on April 18, became angry and said he didn't want to see her again.

Mambor maintained he never had sex with her again after that revelation.

But Ms McMurray presented a text message from Mambor to the alleged victim from the same date, just hours after the alleged conversation.

''[Girl's name] are you going to come back tonight? Please let me know so I can book the ticket tonight,'' the message read.

Mambor subsequently changed his evidence, saying he made the discovery about her age the following day, on April 19 and had initially made a mistake about the date.

The court heard Mambor encouraged the girl to go back to school, stay out of trouble, get off drugs and take better care of her grandmother.

''Mr Mambor, these are things that you would have told a child … I suggest to you that you told [the girl] all those things as if she was a child because you knew she was,'' Ms McMurray said.

''18 years is still young,'' Mambor replied. ''If they don't go to school they don't have a good future.''

But Mr Purnell said caring for the girl was consistent with having affection for the girl.

The trial before Justice John Burns continues.