If the comment had been about discriminatory hiring practices, or even just generally about ageism in the industry, I would have nodded, and probably upvoted.
But he phrased it as the OP being "too old for a FAANG", which centers the fault on the OP.
Besides which, in whichever sense it was meant, the comment was almost certainly factually incorrect. A math PhD with significant software engineering and engineering management experience isn't someone that gets turned away that easily.
I'm the OP, and I do wonder if I would be too old for a FAANG, with the fault being on me. Building a company as an entrepreneur at age 45 (my age) feels very reasonable, and I've seen research that suggests that my age is perhaps even a sweet spot for building a company. In contrast, I think I would find it difficult to work as an employee at a FAANG company, since it's very different than everything I have done so far in life, and most other employees would be younger than me (informal observation based on having lunch at Amazon, Microsoft, Google and Facebook regularly).
More importantly, joining a FAANG company would mean completely giving up on my longterm goal to massively fund Sage development, and that would be very hard for me to do.
While most other employees at any tech company would be younger than you, is having lunch at AmaFaceGoogSoft really so different from having lunch in a university campus cafeteria?
I am not trying to change your decision, BTW. Your long-term goal is an admirable one and I think achievable, whether CoCalc remains a private company, goes public, or is acquired.
But he phrased it as the OP being "too old for a FAANG", which centers the fault on the OP.
Besides which, in whichever sense it was meant, the comment was almost certainly factually incorrect. A math PhD with significant software engineering and engineering management experience isn't someone that gets turned away that easily.