I'll be the lone voice of (sorta) dissent.
You'll see ageism more in seed/Series A start-ups with brogrammers (but do you really want to work there anyway?). Ace the whiteboard, get 95% on the timed online coding test, get along well with everyone, and then get told that "It's not a culture fit."
Of course, it could be that you won't work 6 days/week without a decent equity stake and 23 year olds are excited! to do that. I suppose that's "lazy" in a way. I'd say it's "not being a sucker."
A lot of interviews are algorithm heavy and the real truth of the matter is that few developers are working on optimizing B+ trees. These types of interviews favor recent grads, which you'll be equivalent to even if you're 37 simply because your algo class was 6 months ago. There's an entire industry around helping software devs prepare for interviews (note: It's not about making them better developers. It's about preparing for the interview.)
In summary, of course there is ageism. It might be related to the fact that you won't be hanging at the bars with them after work because of your kid, or it might be because they aren't comfortable being a boss to someone 10 years older, or it might be because they agree with Zuckerburg that anyone over the age of 30 isn't any good at software (he's 33 now), but it exists.
That said, given the lack of developers, it's a candidate's market and many companies, even if they were inclined, can't be super picky.