First-time buyers make up a larger chunk of the housing market than the real estate industry has generally thought. Forty-seven percent of purchases in the past year went to first-time buyers. Their median age was 33. By contrast, surveys from the National Association of Realtors have indicated that first-timers account for about 32 percent of all buyers.

The difference between the two surveys may stem from their methodologies. The NAR has used a mail-based survey for its annual figures, while Zillow used an online survey that might have generated more responses from younger buyers. The Zillow findings might help to explain why there is a shortage of listings on the housing market. Unlike move-up buyers, people buying their first home don't have a property to list for sale, depriving the market of inventory.

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Adam DeSanctis, an NAR spokesman, noted that his organization's survey out later this month will show a rising share of first-time buyers, although it remains below the historical average of 39 percent since the organization began tracking this figure in 1982.

DeSanctis also noted that government figures show home ownership among young adults is at its lowest level in history, which is why his organization is skeptical that nearly half of sales go to first-time buyers.