Well, it's complicated. In Mario&Luigi: Partners in Time, Mario and Luigi meet their baby selves when they go back in time to save princess Peach. There they meet baby Bowser and he already has just as big ideals as in his adult years (except his ideals are more focussed on food than usually). Though, that's technically baby Bowser and not Bowser Jr, so I'll cut it some slack.
*gets hit by Bowser Jr.'s fire* Aaah!