My son went through major regression at nearly four. Went from having a lot of words to having none at all. No longer answered to his name or made eye contact, became fixated with wheels on toys and other spinning objects, forgot how to feed himself

My 3yo went through a major regression right around 2yo. He went from developing typical language to no language at all. Like said before each child is different. But regressions usually shows a point of "loss" of newer gained skills.

It is usually visible... my 9yo starts cycling back to old behaviors or his speech will slur. It is different with each individual on the specifics.