So, if everyone recalls, Danny was only 14 and thus a freshman when the first season came out. By the end of the series, he had gone through one summer vacation, and was in his second year of high school. Thus a sophomore, with at least a half a school year left. It's not unheard of for teen guys to grow dramatically over their summer breaks, and Danny still had about two and half years of school left. It would be totally plausible that Danny could look like this by his senior year, but I want to say this is his junior year, cause of Dash. (Based off the original tradition of only awarding varsity jackets to upper class men, I'd assume Dash was at least one year ahead of Danny, and had to retake one or two classes. Or something. Which would make him a Senior during Danny's third year...)
Soo, if I was to write a story, I'd go with this being Danny at 17 on his first day back from summer. Which would still be one hell of a dramatic change, as emphasized by Dash. lol The question of course be HOW Danny did it? (I can think of a couple of ways...)
I LOVE the strong bold lines, and how seamlessly you've mixed your style with Butch Hartman's. You've done a phenomenal job making Danny look older, more mature, and more confident with his facial structure while still looking like himself. His body language is overflowing with confidence, and all that muscle mass is perfectly proportioned. Plus the subtle differences in his build versus Dash's helps to imply his more practical growth and development that's better suited to catching ghosts. Makes him look like a fighter.... or a wrestler.
Looking back at your first drawings of Danny Phantom, it's clear that Danny isn't the only one that has grown and matured in this image.
I still hate the goatee though...