Hi Judy, I can't comment on using P Pro with fry but if there are flukes in the system, either skin or gill, because they are generally found together, they won't be too worried if the fry are 2 weeks, 4 weeks or 6 weeks old.
I see no reason why flukes looking for a host would wait till the fry are 4 weeks old to attach themselves. They, would be a host at any age.
Graham