(I beat steel mechorilla btw guys! Now New Fassad is my kryptonite...)
Anyhoo, there's been this theory ever since M3 came out, where many people think Jeff is Dr Andonuts in this installment. I actually believe that theory in some capacity, and heres why:
Similar to Pokey, Jeff might've been time traveling through different eras, and it took a toll on his age and body. However, his symptoms aren't as bad as Pokey, where he is legit wheezing after every sentence, and that he's thousands of years old. Now, what I like to think is that Jeff was abducted by Pokey after the events of EarthBound, and was time traveling with him. Just a hunch, but I find it interesting.