I'm a bit older than OP - just recently turned 46. I've been a software engineer (professionally) since I was 18 years old when I was first hired by a small mom-n-pop shop.

One would think I could simply plop my resume down, do an in-person interview, show a bit of code I've written in the past, plus my github, and that'd be enough. Alas, it isn't.

I've had good interviews that used a whiteboard, and bad ones that did. Overall, though, I detest whiteboard "challenges" and I specifically avoid them. Currently, with my use of recruiters, I tell them specifically not to send me to such interviews if they use a whiteboard (it would have to be something really unique for me to consider it - maybe for a ground-floor startup opp, or something in robotics or AI, etc).

The best interview I had that used a whiteboard was basically where they asked me to write fizzbuzz whiteboard style. Whenever I am given such a task (ie - write code), I ask the interviewer if they mind if I use "pseudocode" - just to get the whole "wrong syntax or keyword" issue out of the way. I've never had a turn-down from this ask. I liked that they wanted me to show I could do fizzbuzz "from memory" because it would show I wasn't copying/pasting from that github repo of "all versions of fizzbuzz", and it would also show I had some idea about programming. After that, and the in-person portion of the interview, they gave me a take-home challenge to write some piece of small software (IIRC, it was a random-number dice game or something), and upload it to their system for evaluation. I actually ended up getting an offer of a position from that job, but I ended up taking a different position with another company.

The worst whiteboard interview I had, though, felt like a complete grilling session. It started off reasonable enough; ushered into a large conference room, and I was questioned by a couple of programmers on their team, plus their hiring manager. All seemed ok. They asked me to do some whiteboard work - some SQL coding IIRC, among other things - all was going ok as I was writing down an answer, but every time I turned around to the interviewers - there were more people in the room. By the end of it all, it felt like the entire staff of the company was in that room and nobody was out doing their job. Had to be 20 or 30 people in there. To be honest, it rattled me - mainly because it was so odd.

I didn't get an offer on that job - and to this day, I am glad I didn't. While the company and the office location all had a "hip and upcoming" startup-like vibe, plus open-office floor plan, etc - at the same time, I wondered why they had such a large SWE team (10+ people from what I recall) for what was essentially a basic PHP CRUD application.

It was interesting to note OP's idea of it being a potential age filter; I'm not sure I agree with that fully. I wonder if it isn't meant as more of a filter for those who didn't go to school to learn the craft. I mean, I've probably done at least once certain things being asked for - but if they couch them in terms that are "defined in the literature" (this also covers asking about "patterns") - I'll most likely be lost. Because I don't know all of that terminology, or what it applies to. I've been coding in some fashion or another since I was 10 years old, but I haven't gone to school for it (aside from a couple of community college classes for C/C++ - algorithms and/or patterns were not discussed).

If so, it's kinda on them because they didn't read my resume carefully; I note up-front that I don't have that education, that I am a self-learner, and that I don't like to be a "specialist" in a particular language or framework-du-jour. Rather, I'm a business problem solver - the ultimate choice of how that problem is solved is an implementation detail that has only a certain bearing on the problem solution. Mainly, it's better to come up with a solution and then choose a language for the specifics of that solution that will work to implement it. 9/10 times you don't need the latest language or framework for most problems. It's more knowing when you do, rather than just picking one and sticking with it forever (ie - I don't want to become someone mired in only using and knowing COBOL). Likely, any problems that crop up in a solution tend to do with how the solution was implemented, not what language/framework it was implemented in.

Lastly - something I have noted post-interview is the fact that seemingly none of the potential employers care to contact or do contact any references you give them. You can ask them if they want/need references, but even those that do, never follow up on them. I am not sure why this doesn't happen - maybe it's just a simple constraint of time vs number of resumes/interviews? Or maybe it's due to past candidates gaming the system, and making it an unreliable metric to the process?