I guess this is the point where us females crawl out of the woodwork...
I'm not a twenties-something (er, forties-something) but I am female (and in the UK). What I noticed, when I started programming, was that at college and when I started work, the male female split was 40/60. But the women who I trained with and later trained-on-the-job, all went into project management, and did not share my technical talent/interest. I can't think when I last met another woman programmer, but it must be over a decade.
When they had children, they all went into "Resource Management" or left and went into education (you try working around 12 weeks school holidays).
But the teenage girls I talk to now don't seem to think that programming is an option for them, they don't think they're clever enough, it's seen as too 'geeky' and, as one famously said to a politician, "just look at the boys who do it". The exception to this is my daughter (who knows how rewarding I find it), so perhaps we are lacking role models?
Admittedly, I'm as guilty as anyone else, the thought of going into my local school and promoting this as a career makes me want the ground to open up and swallow me... But I do think that as a profession/industry we miss potential talent, and a different approach to problem solving, by losing the girls.