I haven't heard of it until today so I had to look for it in the internet...
Maybe it is going to replace the engagement ring, because:
1) both sides of the relationship can wear it,
2) nowadays many relationships last for years without even planning the engagement or marriage. 
Anyway I haven't heard of it in my country, but who knows? Many foreign customs are adopted in the times of globalisation. 
Still I haven't noticed anything on my son's and his girlfriend's hands last time I saw them  ;-)

In my country wear ring only when they get engaged

In Italy the ring means to be committed to someone. Two people just exchange it when they're engaged. 

It depends because everybody is different especially in the United States because our country is so big. Because I just graduated from high school a few years ago I definitively say that some tweens, teens and young adults (and I mean young) might wear "friendship rings or bracelets" to show affection for their girlfriend/boyfriend. However, when people actually reach an age where marriage is a serious option people don't start giving each other rings unless they're proposing. 

Last mounth I wached a reality show and there a Dutch guy had given a commitment ring to an American women. The show name's 90 days to wadding.