While I am not a doctor or health professional, I certainly can refer, in general to this topic. When a boy or girl reached college age, there was often a (very real) phenomenon called the "Freshman 15." More recently, it was found that this was likely a final growth spurt before an adolescent finally reached actual adulthood.
Having said this, with a hard-driving young athlete, there would still need to be a balance between training, hormones, and diet that would make it possible to get the results that one wants in weight. However, that is not always possible.
Why?
In creating a final adult body, one has to remember that there will be endomorphs, mesomorphs and ectomorphs.
As an example...Rowbury, Linden and Purrier are built completely different. However, they are all fast, talented and are comfortable in the adult bodies they have.
A coach or trainer that does not factor in body type and tries to make a mesomorph into an ectomorph may be doing permanent harm because a body type is mostly genetic.
In ectomorphs, weight can drop off. It's different for mesomorphs and nearly impossible for endomorphs to have a "drop"
So, what it comes to really is the proper training for the proper body type for the proper results.
This is what is lost when you just focus on weight or the appearance of weight.
Without assessing body composition to know what someone will be, you can't know what someone should look like, especially as an adult.
The takeaway here...
Training alone is never an accurate gauge of the results of training.