First-time buyers may be entering the U.S. home market in greater numbers than industry watchers had assumed.

Nearly half of sales in the past year went to people who were buying their first home, according to a survey released Tuesday by the real estate firm Zillow. That's a much higher proportion of the market than some other industry estimates had indicated.

Zillow's survey results suggest that this year's growth in home sales has come largely from a wave of couples in their 30s, who are the most common first-time buyers. If that trend were to hold, it could raise hopes that today's vast generation of 18-to-34-year-old millennials will help support the housing market as more of them move into their 30s.

That's among the findings in a 168-page report by Seattle-based Zillow. Its survey also found that home ownership is increasingly the domain of the college-educated. And it indicated that older Americans who are seeking to downsize are paying premiums for smaller houses.

Here's a breakdown of Zillow's findings: