My experience has been that many women drop out of development in their late 20's/early 30's, often to have a family. Of those that do come back, many go into a non-development career track including management, support, testing and tech documentation. Many want a part-time role which doesn't really work well with development.

At one point I had two managers, both women in their late 40s, and they used to time-share the one job. One worked Mon-Wed, and the other Wed-Fri.

I do know one female developer who has worked non-stop and is now in her mid-30s, and shows no signs of dropping off that. But she has no kids.