I think that it’s as unlikely that Johnny will permanently “grow up as it is that Bart Simpson will–and that’s because within the narrative structure of FANTASTIC FOUR, being the young guy is his role. And whenever he’s portrayed as "grown up” for too long, he inevitably becomes boring–he no longer has an active narrative place within the proceedings. In some ways, this is like asking for the Hulk to come to terms with his anger–you could do that story, but it’s not really good for the character or the series.