army mandatory in korea?

Mandatory to enlist in some military service or civil/public service.  Here's an article about actors heading out in 2017 and it explains it a bit. 

https://www.dramafever.com/news/korean-celebrities-who-will-enlist-in-the-military-in-2017/

I think it's pretty much just like in other countries where it is a MUST for boys. Officially you have to do it, unofficially there are tons of ways to get out of it. Most come up with "medical reasons". Maybe it's like this in Korea too. 
What I find more interesting is that apparently you have a bigger choice on when you want to do it. Some do it right after finishing school others go in their early 20ties. Or maybe it's just how actors do it