Here's how I look at it. It's not the decimal number of years since your birth, but the phase in your life that closes doors.

In tech, you constantly need to keep up to date on the latest technologies, read publications and go to conferences, because experience gets stale very fast. It's also easier (than in fields like medicine or law) for junior techies to get up to speed in experience, because they can skip the decades of now obsolete tech and fast forward to roughly where you are at. This is fine. It can be fun and it's definitely rewarding.

Then by your early thirties, three things change at the same time: you get promoted to a busier, more responsible position, you have small kids, and you get fat. The (waking) time you have for yourself shrinks from 8-10 hrs to about an hour if you're lucky. You can use that hour to study and keep up to date, to do something about that gut and exercise, or you're so tired you just want to relax. Pick one.

In my opinion, this is why middle aged people seem so lazy and boring.