In most U.S. metros, it is now more affordable to own a home than it was 15 years ago, according to a recent Zillow analysis of third quarter income and home value data.  

It's also now more cost effective to buy rather than rent, on average, by a pretty good margin. Home buyers can expect to pay 15.3 percent of the area median income (AMI) on mortgage payments, compared with the 29.9 percent of AMI renters can expect to pay, nearly twice as much. 

However, saving up for a modest down payment can be a big hurdle for many renters. With more of their income going to rent, as well as other debt payments such as education and/or car loans, it can be hard to save for even a 3 percent down payment required by certain FHA, Fannie Mae, and Freddie Mac loans.

In areas where a median home price tops $450,000, a 3 percent down payment still comes to $13,500. Yet, millennials have an average of only $3,000 in non-retirement savings, a recent Demand Institute study found. 

However, as renting grows less affordable and buying conditions become more attractive, Zillow expects 2015 to be a big year for millennial home buyers, who could surpass Gen Xers on their way to becoming the largest generation of home buyers.