Ageism exists in the professions but it's legible and practically published. If you start law school at 30, you probably won't get Biglaw unless you clerk for a Supreme Court justice. You can get into a PhD program at any age, but you're not going to get a tenure-track position if you're after 40. Midlife career switchers generally don't get in.
The difference is that, in the professions, you have to get in early but it's the norm to move up fast enough that ageism isn't a problem because, even if you make a couple of mistakes, you'll be at an age-appropriate level. You may not be a biglaw partner or chief surgeon earning 7 figures, but you'll be substantial enough that you're still taken seriously.
In software engineering, there isn't a well-defined sense of what "up" is or what's "age appropriate". There isn't a published career track and a legible ageism. It's there, but it's hard to tell exactly when it's there. Is being a programmer at 55 age-appropriate? If you're an AI researcher at Google X, then yes, absolutely. If you're checking your Jira every morning to figure out which user stories you're going to be working on, then no one's going to believe that you chose to be a programmer instead of a manager. (Hell, I wouldn't believe you. I might still hire you, because I'm mature enough to separate low social status in one theater apart from low ability. If you're 55 and still have to deal with user stories, it means that you managed your social status poorly; but you might still be a rock-solid engineer whom I'd hire in a heartbeat.)
If you look at the Valley's emerging professional model, it's not a kind one and it's not one that ages well. You choose between (a) a "main sequence" where each jump is a dramatically different job, from engineer to manager to founder to investor, and where there are structural reasons why most people will never make it; or (b) fighting for the small percentage of jobs that are genuinely interesting and age-appropriate at any age.
I think it's much easier to deal with age if you're a consultant, because it leaves you out of the political structure of a firm. It makes people uncomfortable to have a 35-year-old "Software Manager II" overseeing a 55-year-old badass. With age, you just don't fit into the corporate hierarchy unless you've climbed it. If you're a consultant and live outside of the hierarchy, then age doesn't really matter.