solidcore is one of the hardest physical things I've done, and as a former rower, I've done some hard things. OTF, while not objectively easy, is much easier to modify into a doable form than solidcore is. Even at its "easiest" level, it's still super hard.
As others have said, it's about strength, and anyone can build that. I think it's best for people who can at least hold a plank on their toes for a minute; less than that, and you should work your way up before shelling out. Just my opinion, you can certainly start wherever you are, as long as you're willing to be humbled and to work hard.