Carry over skills?

If you have all the milestones (that I've found) when ageing your infant up, then you start with level 2 in Thinking, Communication and Movement skills. I guess that Gross Motor maps to movement, Fine motor maps to Thinking, and Social maps to Communication. I haven't experimented with how fewer milestones maps to lower skill, but I may do at some point.
(link is to the post where I list the milestones)
https://forums.thesims.com/en_US/discussion/comment/18245972/#Comment_18245972