This informaton that in the last few years far far more slightly older girls are getting married and fewer 19/20 year olds is based on hundreds of conversations with shadchanim. an exact study of girls schools has not be done yet to that effect because the studies focused on girls 24-30.
B”H in the last few years girls there has been a significant increase in shidduchim for girls 24+.
as for your statement”more of them should get married at that age”
not if you want to solve the shiddcuh crisis they shouldn’t….
any individual girls has a right to try to get married, but hopefuuly more and more the boys will continue to lean towards the slightly older girls.