It's available now.
It's only available in big cities and it's far from reliable. Verizons 5g will roll out much slower that T-Mobile b/c Verizon has mmWave and T-Mobile doesn't. I think AT&T is a slower version of 5g as well. You should check the coverage map and see if it's worth it to you.
Mobile providers are working on that technology.
According to what I heard on a TV-news program, 5G is available in some areas.
In my opinion, I think that we are going to remain on 4G until the big provers such as Comcast, Verizon and others are able to update to 5G.