When you've worked on 100+ projects (and accumulated a few pathologically obtuse, worst-case horror stories), you realize it wasn't bad luck, but those landmines inevitably lurk in every younger stack.
Maybe you get lucky and don't step on one. But that doesn't change the overall risk based on their existence.
As Torvalds said in the spinlock back-and-forth: it looks simple, until you're looking back over two decades of patching edge cases that you never saw coming without the benefit of hindsight.
And having that experience makes you look forward very differently.
(At least where production, must-work code is concerned. Go nuts with toy / personal / experiment projects!)
You're conflating age with experience. Yes, somebody that's younger is likely to have less experience, but the two words are not interchangeable. Their link is definitely not strong enough to begin guessing peoples' ages based on their level of expertise.
The first time it happened, I was like "it's fine, we can just rewrite modules A and B". As the years go by, I see it more and more. And now being a freelancer and having been in about about 15 projects, 1/3 greenfield and 2/3 legacy, I see a pattern.
Use the most appropriate tool for the task (taking into account not just the tech itself, but also the market, the maturity, possibility to find developers, etc), not the "hot tool of the day".