I've only the one DD, but am curious about her cousin - she's 22 months and can already count. I don't mean reciting her numbers - if you ask her how many ducks there are in the book, she points and counts, 1, 2, 3 (only goes up to 3). She's been into shape sorters and puzzles since about a year (DD is 16 months and is not interested in these things at all )
She does have a much more structured life than my DD (creche, parents who play with her constantly) who is pretty much left to get on with things (we read books with her, talk to her, and do play with her sometimes, but in a very unstructured way IYSWIM). I wonder if these things matter - I'm of the mind that they develop in their own way and that you might be able to push it on slightly, but as with DD and the shape sorter, you can't lead a horse to water etc. OTOH my DD is a very chilled easy going child and DN is very high strung, likes things just so etc.
I suppose I'm wondering if I shouldn't be making more effort to teach DD things (as I say, she is 16 months, can say 100+ words, use a spoon and cup, etc etc so I'm not at all worried about her, but she certainly can't do some of things that DN could do at her age.) rather than just letting her potter and footle about