Landlines are becoming relics of a bygone era.

About two-third of millennials live in a wireless-only house, according to the preliminary results of the National Health Interview Survey, conducted by the Centers for Disease Control and Prevention.

Among adults aged 25 to 29, 66 percent live in a house without a landline. Among the 30 to 34 age group, 60 percent live in a house without a landline, according to the survey.

Related: Mobile Technology for Business Owners On the Go

The percentage of millennials living in homes without landlines is higher than for adults in the U.S. overall. Nationwide, approximately two in five households (41 percent) live in a wireless-only household.

While the percentage of adults living without a landline continues to increase, the rate by which it is growing has slowed over the years. The number of wireless-only households rose 2.8 percent from 2012 to 2013, and 4.2 percent from 2011 to 2012. Five years ago, annual growth was 5.2 percent.

Related: More People Want to Get Shocked Than Sit in Total Stillness for 15 Minutes

The trends of wireless-only living also tend to correlate with home ownership. Of those adults living in rented homes, 62 percent had only wireless service. Among adults living in a home owned by a resident, only 29 percent lived in wireless-only homes.

In addition to whether an adult owns or rents his or her home, part of what could be affecting the growth rate in the number of adults living with landlines is cable connectivity. Many cable packages include a landline phone, and so many adults who have presumably been set up with Internet and television service are also getting -- or have already had -- a landline installed as part of the package.

Related: Supreme Court Unanimously Rules Police Can't Search Smartphones Without a Warrant