In your thirties, you're more likely to have a stable relationship and kids to care for.
If the tools you learned in your twenties age out of relevance, then you may well have a hard time retraining to new tools while raising small kids.
If someone learned React in your 20s and then "aged out", they must have been an early adopter. React is only 9 years old and most developers didn't start learning it in 2013. And it's far from irrelevant: https://trends.builtwith.com/javascript/React
> "Modern" web frontend work involves a lot of chasing after new tooling, frameworks, and libraries.
Le meme never gets old.
What if you learned classic AngularJS, due to go unsupported in a month and a half? What if you threw in with ExtJS ages ago and the enterprise corp you've been using it for just downsized and outsourced your project? Maybe you were an EmberJS aficionado but can't find new gigs using it. I worked on a greenfield project in Backbone/Thorax/Mustache a number of years back but haven't seen any of those in the wild since. Maybe you love Elm but can't find any places that will hire you to use it. Ditto using Clojure on the frontend.
Picking the winner is a great trick, but my point was that if you didn't (or just the companies you worked for didn't), you may find yourself in a bit of a hole, specifically if you have a lot of other responsibilities in your life.
To be clear, I'm not thinking about people who've used lots of languages and tools here. I've done time working for huge corporations for whom software is a cost center, and it's easy to find people in those contexts who are competent with the tools they know but have narrow enough experience that moving to something else is difficult for them.
The "average Joe" programmer's experience is what I'm thinking of here.