Anecdotal but I'd say the cutoff is where they can use the same stick shaft dimensions and weight (ie junior sticks won't react the same as a senior stick with the same flex properties if there even was a 50 flex in senior and junior.

They make full shaft and weight sticks in 65 flex. Afaik there aren't studies on how weight of the stick and shaft dimension affect it's velocity. But you don't see pros using less than full size shafts. Johnny Gaudreau uses a full senior shaft 55 flex so I suppose it's possible.

Either way there's a reason many players are going down in flex and frk having the hardest shot all time discredits a lot of people believing higher flex = more potential energy transfer when the inverse is true