As for your nephew, there is ample info on this site that touts the benefits of starting small, and in particular starting at 500cc or less, except in certain cases of prior experience and certain models of bike that are more forgiving than a race-tuned 600.
As for yourself, I think enduros are a great choice for most types of riding. One thing you’ll want to consider is whether the vibration that often accompanies an enduro thumper motor will be an issue for your arthritis. Every model is a bit different and there are lots of things you can do to reduce vibration, but it’s something to be aware of.
On the positive side, enduros typically have a very neutral riding position, meaning that you are neither leaning onto or pulling back on your hands.
My advice, go to a bunch of shops, sit on lots of bike and start getting a feel for what feels good.