Teen Wolf's birthdays are all over the place and no one can confirm on Isaac's age during the show. So that's what I'm going to be trying to do.
We know he was either born September 22nd in the The Hunt game or February 27th on the official calendar. However, since the calendar has gotten; Stiles, Lydia, and Allison's birthdays all canonically wrong then we have a winner; his birthday is canonly until otherwise September 22nd.
Now for his age, seasons 1 and 2 share the same year as we know season 3 starts with a four month gap and a new school year since Boyd and Erica have been labeled as missing for four months and 1 and 2 were tied together. Since in season 1-2 they are sophomores/in grade 10 that would mean the average student besides Stiles, Lydia, and Allison would be 15 or 16. We don't see Isaac driving until the hospital scene with the alpha pack in 03×10 meaning he most likely gained his license or just hoped behind the wheel either way because there was no one else. Since it seems you have to be 16 to get a license when Erica says; "I just turned 16 over a month ago, I wouldn't mind getting my liscene." Proving she's unable to before hand, or at the very least allowed a full one. She could always have a permit and that's what Isaac could be using since there teninchally is an adult with him- Peter.
So it puts him between 15-16 between all three seasons since we never see him drive again I'm assuming then he just has a permit by that since we see everyone else drive constantly if they are of age; Scott, Allison, Lydia, and Stiles. And most people believe season 3 takes place between August or September and since we have confirmed Isaac's birthday is September 22nd, this canonly places him at 15 throughout seasons 1-2 and turning or just turned 16 during 3A. Meaning he is around the youngest in the group besides possibly Boyd.
That means we can canonly tell, Isaac Lahey is born on September 22nd and throughout his time on the show he is of the age of 15 in seasons 1 and 2 and 16 in season 3A and 3B.