Now certainly DiCaprio's work as Luke Brower-Seaver, the show's
Cousin Oliver, wasn't on the same level as the quality of work he would later go on to do, but he was already clearly a talented and committed actor. Just one year after
Growing Pains was canceled he was astonishingly good in
What's Eating Gilbert Grape, so I think it's safe to assume that he was already an obviously better actor than Cameron at the time they worked together.
So for at least one year of his professional life, then, Kirk Cameron was confronted, regularly, by an example of what a real actor his own age should look like. And yet he spent all that time on the same set with and in the same scenes as DiCaprio without apparently learning anything — without even seeming to realize that he
needed to learn anything.