Is kite-boarding a thing one can rationally get into in middle age?

I’ve spent a lot of time in Palm Beach watching kiteboarders (my mother in law’s condo is at Lake Worth park which is a prime spot). I’ve gotten to know some of the regulars and guy who teaches lessons. Barrier to entry is so high. The actual physical part of it is difficult. Equipment is difficult. And then there’s the wind. Watching newbies who can’t get out past the shore break because of the direction the wind is going is painful to watch.

But… it absolutely looks amazing and I’ve been tempted to give it a shot as well

Yikes that seems like a very high skill/high physical toll activity...have you considered pickleball instead

It depends on you. If you are used to the water, a lot of experienced waterman say you can't always surf, you can always wind surf, but you can always kite. The people saying high cost to entry must be comparing it to a surfboard, because $1500 for the rig and another $1500/$2000 for the kite, it is high. However, it is pretty cheap to go anywhere you want to, anytime, and not need a boat. If you wakesurf here it's $80k to get started.
Most of it is based on wind and body weight. As a beginner it's best to buy the biggest kite because it works with light wind and heavy weight. Learning the rigging for the take off is the toughest part. Not getting all caught up. They actually had some classes at Fountainbleu on the North Shore. It is definitely something you can do.