ames
In every developed society that does not artificially control birth rate the birth rate has gone up dramatically over the past hundred or so years. This is due to many factors including reduced maternal mortality, reduced infant mortality, a dramatic increase in life expectancy, earlier weaning due to better nutrition and the development of infant formulas, and fertility treatments.
In the frum world the general trend is the further to the right community is the higher the birth rate. As there has, over the past several decades, been a general shift to the right it is reflected in the birth rate.
Anecdotally it would seem to me that people are marrying earlier. When I compare people I know in my grandparents, parents, and my own generation that definitely seems to be the trend. If anyone has any hard evidence on this point I would love to hear it.