I'm sure it is myelination. Apparently the slower your neurones myelinate the slower you develop (would make sense) so certain pathways have to be myelinated before you will be able to dvelop language etc. This is VERY rusty so I may be a little off track- but sure I am vaguely right. The longer your neurones spend unmyelinated the longer they are exposed to potential "attack"- not the right word- but they longer they are vulnerable to being sent off course.
I remember years ago when I was pregnant with ds2 chatting about whether we were going to vaccinate or not. And she was saying something along the lines of being more cautious if it was a boy (both her dd's are fully vaccinated).
I found teaching teenagers that girls were vile between 13 and 15 and then began to become human again whilst boys were pretty awful (and monosyllabic- saw that so often!) between 15 and 17 and then became lovely again. They definitely lagged behind. Both were good fun by 18! Luckily I mainly taugth 18/19 year olds