The first children of families were used to success their families, as a result, they usually have been educated more carefully, and then they are more educated than their younger siblings. But when time passes by, things are changing. Succession is not exclusive, and children have equal right to be educated now. Do the former successors have equally opportunities to get higher degrees now? No, younger children are more successful to get higher education; not only because the parents are more experienced since the birth of the first children, but also because younger children can benefit from their older siblings.
Again, in general, more skilled parents and sometimes a role model sibling in are two major differences between a first child and her/his younger siblings. They lead to better education quality and usually higher education degree for younger siblings.