so he was 16 in season 1-2 and then 17 in season 3-4 and 18 in 5-6 ?

He was a freshman in season 1 so he was 14 then 15 for season 2 season 3 he was 16 and season 3-4 he was 17 and 5-6 18

The show starts in the second half of their Sophomore year (January 2011) when Scott was 16 and season two starts immediately after in the last full week of February 2011. I could be wrong, but as far as I recall Scott's birthday was not specified.

Season 1 & 2 - Sophomore year, spring 2011 semester

Season 3 - Junior year, fall 2011 semester

Season 4 - Junior year, spring 2012 semester

Season 5 - Senior year, fall 2012 semester

Season 6A - Senior year, spring 2013 semester

Season 6B - A number of months after graduation

Plus a two year time jump for the last episode (most of the episode is a flashback/Scott's retelling of what happened)

You can also check out the Timeline page or, for a visual representation, the handy calendar I made for S1 - S4 back in 2014