I think it starts very early. You can find threads here from time to time from people saying things like, "my son's coming home saying women can't be doctors, even though he's only ever seen a woman GP." Plus some toy shops are incredibly gendered, pink and blue aisles, dolls and Lego. And then you get parents who say, "Oh, I was never any good at maths!" - it seems to be more acceptable to be bad at maths than at reading. Probably any one of these things wouldn't be an issue, but when you're getting a whole load of separate messages about the same sort of thing, it all reinforces it.
There's evidence that ideas about STEM subjects not being for girls can be set in by the time children start secondary school. I went to a single-sex secondary school, and I think that helped massively - we were told we were the business women and academics of tomorrow, and I was really taken aback in the 6th form when we were planning a joint trip with the boys school to France, and we girls were told we would be in a big shared tent rather than separate tents like the boys, and I just couldn't understand why, just because we were girls (not least because I was probably more adept than most of the boys at pitching a tent.) I have often wondered since whether if I'd been at a mixed school all the way through, I wouldn't have questioned it, because I would have been more inured to it.
Things haven't necessarily improved - there are still some teachers who don't seem to think women can do as well in technology, and while they're in a minority, if they're the one who is teaching your daughter, it can be a big influence. I hope in a few years, with the changes in the curriculum from last year, things will improve - but last year, there was a BBC programme about girls can code, and the young women on that, they'd never even considered technology as a career - it wasn't that they'd thought about it and decided against it, it was just never even on their radar.
I've been working in IT for nearly 2 decades now. I've always been in a minority, and for some years, the only woman in the department. I've had outright blatant sexism about women not being as good at tech (oh, they are, because they usually have to be that much better,) and loads of low-level stuff, just signs like "men working behind doors", and stuff that's really well-meant, but still makes you feel "othered", like when I walked into the reception of a training site for a techy course and was cheered, just for being a woman. It can be draining - my male colleagues never have to deal with crap like that. But the more women who decide it's a good career for them, the easier it will get.
Some companies are definitely better than others, though. Mine has a good women's network, which puts on some great support for professional development, and gives opportunities you wouldn't normally get in your day job. I've been to an interview with another company where it really wasn't a woman-friendly feel, and I've recently chosen not to continue with a job application for a position I'm a great fit for, but they've got one woman on their board and in senior management (plus the package wasn't enough, but it was the lack of women which put me off.) But there should be a lot more women doing this sort of work - the pay isn't bad, it can be interesting and varied, it can be flexible (obviously exact conditions vary according to role and employer) - it should be ideal for women. Plus there are so many roles in technology these days - it's not all PC repair and programming (although those roles are there). There are also loads of different programmes to encourage young people, and girls in particular, to consider technology careers, and we're going to need those people coming into the industry.
And yet I still have only had parents asking me to advise their sons, not their daughters, despite the fact I spend half my life (or so it sometimes seems) banging on about why women should consider a career in IT.