I think that starting earlier vice later might be helpful as you won't have as much muscle memory and habit built into your riding, adapting to the foil might just happen naturally as you figure it out.
I also think there's no such thing as too early, you wrote, "how good do you have to be?"
In order to do what?!
In order to learn while crashing your kite all the time and having zero fun at all? None, anyone can do that today if they want!
In order to learn while crashing your kite sometimes but getting away with the occasional session where you make some minor improvement? Not that good, you can be pretty amateur to do that.
In order to learn quickly without ruining your kite gear and having fun along the way? 12-18 months kiting to the point where you're able to jump, transition both directions, ride really powered up, bar juggle when your lines get twisted, etc.
In order to quickly jump on the foil and ride away your first session without ever touching the water, intuitively mastering foiling tacks and gybes on your very first attempts? I dunno, that doesn't really sound possible, lol.
So anyway....how good do you have to be? None, any rank amateur can start today, it'll just suck and their risk of injury or equipment damage is likely to be kind of high. If you want to enjoy the learning process and feel like you're making meaningful headway, you need to understand how the balance between board and kite control creates power, and be an expert at getting the kite to do what you want it to through smooth, practiced inputs. This took me about 18 months of 'regular' kiting before jumping on a foil, but I was an excellent sailor, snowboarder, mediocre skateboarder, okay surfer, and very strong swimmer; your mileage may vary...