BLUE TONGUE SKINKS

So I recently bought a BTS from a store and when I asked the employees about the age of the skink they all were really unsure and only one had an answer which was maybe about a year old. Anyways my question is how to I tell how old my BTS is?

There is no way to tell the age of a reptile, unfortunately. Most BTS are born somewhere between 6 to 8 inches long, but from there the speed with which they grow has so many variables that it's impossible to tell age simply by size --some babies reach adult size within 6 months, others can take a couple years.
Most of the common BTS species in North America top out anywhere from 18 to 28 inches long depending on the species.

If you tell us how big he is and/or post some pictures, we could take a guess if you want?

The skink is about 15 inches long
Here's a picture:
https://www.instagram.com/p/BPbfmsOjMZ2/?taken-by=headintheclouds99

Here's a video:
https://www.youtube.com/watch?v=afeRif4-VFw

Well, your skink looks like a gigas species for sure, most likely a Merauke, so he may grow some more still. My rough guesstimate would be that he's somewhere around a year old.