Cognitive and linguistic development is something that continues over our whole lives, but at the earlier ages there are a whole lot of basic concepts that aren't in place yet, like theory of mind (anecdotally, most roleplayers who try to roleplay with their young <10 year old kids find that the kids have trouble pretending to be someone else), and there are lots of linguistic structures that have certain thresholds of complexity.

So children pick up new linguistic structures at around the same rate as they develop the cognitive machinery to deal with them, and they try to regularise everything. Adults on the other hand already have all that machinery in place, plus they already know what language is supposed to be like based on prior experience, so a lot of structures are much easier for them to acquire (and interesting, the order of acquisition for adults in a particular language is largely the same regardless of their first language, so it's prior experience with language in general that's important, not experience with similar structures). Oh, and there's some not-yet-repeated results that also say that education/literacy is a big factor in individual linguistic complexity, which would probably have knock-on effects for acquisition.