Shawn M. was 13 years and 10 months old. At that time, her parents were entertaining out-of-town guests. Shawn suggested that she sleep in her family's camper trailer which was parked in the driveway in front of the house. Shawn's parents agreed to this arrangement on the condition that she keep the windows shut and the door locked. On the night of June 3rd, Shawn's father was awakened by the barking of the family's three dogs. He went out the front door and heard male voices coming from the trailer. He opened the door of the trailer and heard somebody remark about his presence. He then heard a male voice say, 'Let's get the hell out of here.' There were three persons on the bed. One of the males, D, jumped off the bed and tried to get out the door. Mr. M. wrestled with him and held him around the throat. The other male, James Garcia, stabbed Mr. M. in the right shoulder. Both D and Garcia then ran away. At trial Shawn claims that she locked the door and windows and fell asleep, but was awakened by D who was knocking on the window and asking to be let in. Shawn said nothing and D. Approximately a half-hour later, Garcia came up to the window and asked if he could enter. Shawn did not respond so he left. D returned and again asked to be allowed in. Shawn did not answer. D and Garcia left, Shawn went to sleep. Shawn was then awakened by the sound of barking dogs and by Garcia, who had a knife by her side and his hand over her mouth. Garcia called to D to come in, and D entered the trailer. Garcia told Shawn to let D 'make love' to her, or he -- Garcia -- would stab her. Garcia gave the knife to D who held it to Shawn's neck and then gave it back to Garcia. Shawn asked Garcia to put the knife away and he complied. After removing some of her clothes and with another threat D proceeded to have sexual intercourse with Shawn for about 15 minutes. During this time, Garcia knelt on the bed and said nothing. The father entered the trailer. Mr. M. grabbed D as he was trying to leave, and Garcia stabbed Mr. M. in order to free D. Shawn testified that she knew Garcia 'pretty well' and had known him for approximately one year. She had last seen him about four days before the incident. She also testified that she was very good friends 'off and on' with D and that during one three-month period she spent almost every day at D's house. At the time of the incident, however, Shawn considered Garcia, her boyfriend. At trial, Shawn admitted that she told both Garcia and D that she was over 16 years old. She also conceded that she looked as if she were over 16. Garcia claims Shawn invited him to spend the night in the trailer with her so that they could have sex. He and Shawn engaged in sexual intercourse about four times that evening. Shawn invited Garcia to come back the following night at midnight. The next night, D and Garcia showed up but were told by Shawn to return at midnight. Garcia knocked on the trailer door. Shawn, wearing only a pair of panties, opened the door and invited them in. She told them she wanted to 'take both [of them] on.' She then told Garcia that she wanted 'to make love' with D first. When Mr. M. entered the trailer, D was on top of Shawn. Garcia denied threatening Shawn with a knife, taking her nightgown off, breaking into the trailer or forcing her to have sex with them. Appellant's sister corroborated Shawn's testimony that Shawn made daily visits to the Olsen home during a three-month period. She testified that during these visits Shawn and appellant would go into the latter's bedroom and close the door. On one occasion, appellant's sister saw the two in bed together. On several occasions, his sister entered the bedroom after Shawn had left and noticed the bedcovers 'messed up' and an odor which she associated with sex. D's next-door neighbor testified that he often encountered D and Shawn emerging from D's bedroom in the morning. Joseph W., Garcia's 16-year-old friend who also knew Shawn, testified that he and Chuck A., a 15-year-old neighbor, had simultaneously gone to bed with Shawn on two occasions. On one of the occasions, Chuck's father caught them in bed and ordered Shawn out of the house. Both Chuck and his father corroborated Joseph's testimony. The court found Garcia and D guilty of violating section 288, subdivision (a). The court rejected defense counsel's argument that a good faith belief as to the age of the victim was a defense to the section 288 charge. D was sentenced to the lower term of three years in state prison. This appeal followed. D's sole contention on appeal is that a good faith, reasonable mistake of age is a defense to a section 288 charge.