bts are considered a five year old group in korea. 2013 + 2014 + 2015 + 2016 + 2017 = five years. so, it seems they were eligible for nomination and got nominated but sadly didn't get chosen within the top seven winners.
I dont think it was about how old the groups are in Korea. But their service length.
That was the list from the previous article about the nominess released before. The third column was the length of affiliation or contribution. And they wrote BTS's as 4 years. That was likely also the reason why Twice got Minister award instead of the prime minister which they were initially nominated for.
It's indeed weird regardless how you wanna see it. Oh well...