Hey, I'm new to reading this story and I got to Authors Note, and it said to message you if something wasn't correct, in the first few eps of Season 1 Stiles drags Scott to the Hale house and Derek appears and Scott doesn't remember him so Stiles tells him that Derek is a few years older then them. So Derek is probably around 18-19 years of age, while they're about 16-17. I hope that helps (n iderk if you'll see this lol)