This is my educated guess on the topic having spent time investigating the intersection of poor memory, human growth patterns etc where this topic intersects:

Norepinephrine is a key regulatory of memory. Before a certain age the body is much more in parasympathetic activation v.s. sympathetic activation, very low Norepinephrine, lower cortisol response needs, etc. Once growth slows down and this can flip over then mammals will start to form memories. Human curves for the curious: https://obgynkey.com/growth-and-puberty-3/

Some fun notable mentions such as many earliest memories are "super scary" and those with poor ability to produce Norepinephrine and slower growth velocity will have later "first memories" than everyone else.