The character ages on Teen Wolf are notoriously difficult to pin down, as the show does not seem to place much emphasis on creating a coherent timeline.

Scott and Stiles are 16 in Season 1 and just starting their sophomore years. The ages are based off the drivers license status and the laws of California.

Stiles already has the legal right to drive in 1x01 - Wolf Moon. California law states that at 15.5 years, a teenager can get a provisional permit, which requires that they be accompanied by a licensed driver at least 25 years old at all times. At 16, teens can apply for a provisional driver's license. This allows them to drive solo, except between 11 p.m. and 5 a.m. or when they have a passenger under the age of 20. At 17, the passenger age restriction is dropped, but the curfew remains. At 18, drivers get a regular license. 

Strictly speaking, the boys should be 17 given how they drive each other around. But it's also known that Stiles has no regard for the law, and Melissa McCall doesn't seem too concerned with the boys breaking curfew, so the likelihood that they would follow the passenger restriction while driving is quite small.

If Jackson has indeed been captain of the lacrosse team for 3 years, as Lydia implies, then this would make him a junior instead of a sophomore and likely 1 year older than Scott and Stiles. If, however, the Cyclones aren't state champions for 3 years running only because Jackson was captain that whole time, then he could be a sophomore with the rest of them.

Jeff Davis has stated in a Q&A that the boys are starting their sophomore year, which is consistent with them being 16.

Derek's age is discussed in more detail on a separate page but it is most likely that he is 22 at the start of the series.