I want to point out your own contradictory comments about absorption and specifically mentioning a typically highly processed food (orange juice), one which has been stripped of its natural fibers and flavors.

That age group (and all others) should be eating real/whole fruit or having the juice fresh (I.e. just juiced). They would be better served getting this advice than creating more anxiety about protein intake.

Is there any reason to think that freshly squeezed juice is chemically different from, for example, frozen juice concentrate?