I agree that it varies based on many things.
Our toddler was able to recognize and point to certain numbers when asked around 12 months (some letters even earlier). By around 15 months she was able to pick out any given number from 0-10. Now at 20 months, we've started teaching her the signs for numbers (it varies slightly from traditional holding-up-fingers, but not much) and she's been using them some. She'll point to the number 5 and then hold up her hand doing the sign for "five." That sort of thing.
But she's still not showing a lot of signs of being able to count objects and associating quantity with a number. 'Course we haven't really worked on it a lot, but we're in no rush.
We have worked on the numeral recognition though, as part of playtime and reading -- same with letters (and shapes and colors, etc.). It's something we all enjoy, and as long as learning is fun, we're all for it. We make a game out of it and she loves it.
Puzzles have been a big help with her ability to recognize letters and numbers. Moreso than books even, I think (since the letters and numbers aren't typically contained in one page, but are spread out). Also we have a foam playmat on the front porch that has the letters and numbers in it. That helped too.
Using ASL with her has been fabulous, since she's not very verbal yet.
But I think as with anything, it depends a lot on the child and the level of interest in whatever it is you're teaching him/her. Guinevere loves learning about some things but has zero interest in others. So it really depends on a lot, but it never hurts to expose them to learning, as long as it remains a fun experience for everyone, I think.