a decreasing trendP < 0.001
The mean ages were 9.81 years in 2008–2009, 9.78 years in 2010–2012, and 9.17 years in 2017–2018, with a decreasing trend over the study period ( P < 0.001).
The mean ages were 9.81 years in 2008–2009, 9.78 years in 2010–2012, and 9.17 years in 2017–2018, with a decreasing trend over the study period ( P < 0.001).