If you read the article, one detail is that the statistic is that 51% of those in this age group "live in households" which have only cell service. So in other words, this is including the kids still living at home. Viewed through this lens, the shift towards cell-only is clearly much bigger than the simple 51% number implies.

They should have told us what percentage of those in this age group, who live on their own, have only cell service. But we can make a decent guess.