Another reason is they just get married earlier. Probably a plot of average age of first marriage and TFR would be negatively correlated.
Different family structures as well. It's a lot harder to take care of children with two adults rather than in a multi-generational household with grandparents, siblings, cousins, etc. Communities and expectations of childhood autonomy are also different - people won't call the cops if they see kids playing without adult supervision in most non-US countries.