Now this might be more artistic license or Wolversceince.
Here are a few possibilities.
Wolverine doesn't seem to age much for one thing and in Old Man Logan he had fought in YEARS. My working theory is that he only aged during the course of those years with his wife because he hadn't been blown up.So say he gets completely blown up or close to it he regenerates to tip top condition so it's like he's staying young because of it. IE it restores him to his best state.
Another possibility the future before Earth died as we learned had some great scientific stuff going on including medicine that extended human life, like allowing Reed and Sue to live for hundreds of years, apply this to Wolverine and maybe his body turned younger.