This question already has an answer here:
This gives an idea from 2008. I'm curious to see how many orders of magnitude it's changed since: Age distribution of Stack Overflow users
I very rough estimate, but at least that many have registered.
There are 128 thousand stackoverflow users right now. Users are periodically culled for users that have only 1 rep who don't return after a few months. This can be determined by looking at the users page, and multiplying the number of user pages by 35 users per page.
If you look at users with 100 reputation or more (ie, those that are relatively 'active') you find there are about 31 thousand active SO users.
Usually there are a small flurry of statistical type questions on Meta just after stack overflow data dumps, so if you bump this question just after January ends you may find someone willing to re-run that age query and give you a new graph.