As the formation of households has slowed, so have the number of first-time buyers fallen from prior levels as recorded from the years 1997 to 2013. 
In the last 20 years, the number of first-time homebuyers has seen a steady pattern of year-over-year loss. As seen in 1997 when first-time homebuyers accounted for 2.1% of all US households, representing 2.13 million households, it has fallen to approximately 1.8 million housing units or 1.5% of total US households in 2017.
According to the Harvard study, 35% of first-time homebuyers in 2017 have never been married; this compares to 23% of the prior year. Married homebuyers comprised 61% of first-time homebuyers in 1997, which declined to 46% in 2013. According to the study, the trends seem to suggest that there is a fundamental shift in the way young households are approaching first-time home purchases. It is no longer necessary for individuals to be married in order to make a purchase of a home with an unmarried partner.
The report also saw the average age of first-time homebuyers changed minimally over the last 20 years. In 2017 the mean age for a first-time homebuyer was 34 as compared to 32 in 1997.
From 2005 through 2013 the number of first-time homebuyers had fallen from a high of 2.2 million to under 1.5 million in 2013. Since 2013, though, the percentage of first-time homebuyers has increased to 1.8 million from the 1.5 million in 2013.
On a percentage basis, first time homebuyers accounted for 43.7% in 1997, which compares to 38.5% in 2017.
Here is the link to the Harvard study:
https://www.jchs.harvard.edu/sites/default/files/First%20Time%20Homebuyers%20Brief%20FINAL_alt.pdf