In my avatar photo at the left, you'll see Mariner XL, it's probably 20 years old.
Here's a link to my For Sale post (from 2019):
https://tinyurl.com/3mzj7jwj
You can see, if well cared for, that a fiberglass model holds up pretty well.
Better than me
I sold it because I had an opportunity to switch (sale money from the XL, plus a little more) to a Kevlar Mariner Max. The older I get, the more I like the "lighter but still the same amount of expedition room" feature. Also, the Max came with a rear bulkhead, giving me one watertight compartment - a safety feature.
I wouldn't pay the additional price for a new Kevlar boat, but if the opportunity appeared to have about the same haul space for about the same price, but with one boat lighter (and still seaworthy) than the other. I'd go for the lighter boat.
That said, I am no Tsunami Ranger. The people I paddle with want more sightseeing and less adrenalin. So boat "sturdiness" is not an issue. Either boat, fiberglass or Kevlar, would work for me. As I approach 80. It's nice to be able to hold the (empty) Kevlar kayak over my head for loading on the car and carrying short distances.
Here's the kevlar Mariner Max in 2023 - both the XL an Max are very old boats. You take care of them, they'll take care of you.
View attachment 16807
I usually recommend keeping the paddles flat to the water for quick bracing. But in this case, The photographer was having difficulty negotiating the camera and I had to keep paddling to stay off shore.