There is a lot of debate about Derek's actual birthday, which has a knock-on effect on how old he was when Kate seduced him.
Although the early pilot script refers to Derek as 19, this is not canon as it was changed for the show itself due to the implication that Derek would have been 13 when Kate got to him. Jeff's later statements, discussed below, also rule out this option.
The remaining choices are between 1988 and 1986 for the year he was born.
Derek is 16 at the time of the fire
- His driver's license states his birthday as Nov 7, 1988
- Derek says that he AND his older sister were still in school at the time. He did not go to college, so this could only have been high school. The time of the fire is 7, although it does not say AM or PM. 7 PM could mean he was in afterschool sports. The flashbacks from Peter indicate a nighttime event.
- Based on Stiles' dialogue, Peter told him that Derek was a sophomore in high school when Ennis bit Paige, making it the 2004 - 2005 school year (Derek turned 16 on Nov 7, 2004)
- At the same time as the events with Paige, the Argents are already in Beacon Hills attacking werewolves and trying to wipe them out under Gerard's guidance. It makes more sense for Kate to be with them and stayed to finish the job than for her to leave for 2 years and come back.
- In 1x05 - The Tell, Kate tells Derek that he "grew up in all the right places," implying that he's changed a lot physically since she last saw him. The physical changes from 16-22 are more drastic in men than from 18-25.
- In 1x09 - Wolf's Bane Kate leers at Jackson, who is 16, wishing that she was in high school again, or maybe a substitute teacher so that she could have a piece of that. The line serves no purpose other than to indicate a sexual interest from Kate in teenage boys specifically that age and imply she had been a substitute teacher in order to do just that.
- In 1x12 - Code Breaker, Derek tells Scott, "For you stupid little teenage crush that means absolutely nothing? You're not in love, Scott. You're 16 years old. You're a child." The implication here is that Derek is talking about himself. He was 16 years old. He thought he was in love. His emotions cost him everything when Kate killed his family.
- His birthday written in numerical form would be 11/7. The number 117 has since been related to the Hales in the episode 4x02 - 117 when $117 million was stolen from their vault. It is also the episode where Derek, having been de-aged by Kate to when he trusted her so she could manipulate him again, returns to his correct age.
Derek is 18 at the time of the fire
- Derek is 15 when Paige dies. This also happens at the same time that Gerard blinds Deucalion and Deucalion starts to form the alpha pack. In 3x10 - The Overlooked we learn that Kali slashed Julia a few months after Derek gave power to the nemeton with Paige's blood. In 3x09, the Sheriff says that the file on Julia would be "over 10 years old." If Derek is 18 in 2005, it will be early 2002 when Kali slashes Julia and there 9 years before the present date. Not quite "over 10 years" but close.
- Derek was in school when he met Paige and in school when the fire happened. 18 is the oldest he can be and have these two things still be true. However, it is unclear how Laura would have been in high school also. This simply makes 18 within the realm of possible ages.
- Jeff Davis said at BITEcon that Derek was not a minor when he was with Kate.
- In 5x01- Creatures of the Night, Stiles sees the initals D.H. on a shelf. It's not proven (so far) that they are Dereks, but if they are it implies that Derek was 17 or 18 (the average age of a high school student in the US).
Textual Evidence versus Apocrypha
In literary analysis, textual evidence is anything found within the work itself. This is "canonical." Webisodes created for Teen Wolf written by Jeff would also be considered part of the text as the author is the same, the actors are the same, and the crew is the same. The same mix of talents used to present the text were used to create the webisodes as well, with the intent that they are an extension of the work. Something like a tie-in novel not written by the original creator or a show writer would not be, unless somehow specified. (Disney's project to filter the Star Wars Extended Universe into the books that "count" and those that don't is them specifying which are canonical.) These become apocrypha--semi-official potential additions to the text. Some literary scholars include apocrypha in their analysis and some do not. Interviews, actors' explanations/beliefs, writers' explanations/beliefs, and all other sources of information outside the text itself are apocrypha.
With regard to Derek's age, textual evidence largely supports a 16-year-old reading. "Largely", because there are some statements, like those from the Sheriff, that are in conflict. The desire for consistency within a story would lead us to assume that the Sheriff simply has a poor memory about these events due to how far back in the past they are (he made a guesstimate and was wrong), if we aim for in "in text" explanation, as opposed to "the writers screwed up." The apocryphal evidence supports an 18-year-old reading, largely on the weight of Jeff's word. As the creator, we expect that he should know this sort of detail, so the tendency is to believe what he says. Even though Jeff routinely gives out false information when directly questioned, it remains a strong desire within any audience to take authorial word at face value.
The phrasing of Jeff's reply to the question also leaves the audience with some questions. "He wasn't a minor, at least in California." The phrase "wasn't a minor" seems clear enough. It means he was 18. However, "at least in California" makes little sense in this context, because 18 is the highest age of consent in the country. There is nowhere where being 18 would make someone a minor. Fans have suggested that this choice of phrase is the result of an incorrect belief that the age of consent in California is 16. In this case, Derek, at 16 "wasn't a minor, at least in California" as opposed to all those other places where the age of consent is 18 and he would be.
This reading requires Jeff to be misinformed about the laws of California. And there's no way to back up this supposition short of asking him what he believes the age of consent in California to be. Perhaps someone will attempt it.
This site strives to use only textual evidence as its source of information, as the text is final in a way that opinions from living humans are not. A year from now, Jeff might change his mind. The text as shown on film will remain the same.