If I understand the summary of the evidence in the Cass report correctly, there have not been formal, rigorous trials on teenage children.
This lack of a clinical trial with children is common worldwide with many long established drugs and is not limited to PBs in the UK. This deficit is changing but the opening of https://pmc.ncbi.nlm.nih.gov/articles/PMC4345947/#:~:text=Abstract,evidence%2Dbased%20therapies%20in%20children from 2015 illustrates this issue:
Safety and efficacy data on many medicines used in children are surprisingly scarce. As a result children are sometimes given ineffective medicines or medicines with unknown harmful side effects.
This means we may only know what the main effects of PBs are for all ages and side effects just for adults (children may suffer the same ones to a greater or lesser extent and potentially others).
Because of the potential bias in the non-trial clinical notes (those who can't tolerate PBs may be less likely to return and disclose side effects), even if they were made available to Cass' team, a trial may still have been necessary.