When Wolverine regenerates, why does his hair stop growing?
EDITED TO ADD:
In Avengers vs. X-Men #3, he goes from this
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With a normal person, each individual hair grows for a while, then stops growing, then falls out and a new hair starts growing in its place. The maximum length it will attain (if not cut) varies from person to person, and from place to place on your body.
Wolverine's hair works the same way, but its maximum length is fairly short. It grows quickly enough (but stays short enough) that he finds it pointless to get haircuts.
Well when he regenerates from some physical trauma there must be some kind of imprinted image of his body stored ...somewhere (in cells?). In Iron Man extremis storyline it was stated that human brain (one specific part) stores a image of the entire body. Through extremis technology it is hacked and rewritten to gain superhuman abilities. I am not sure how much this is based on real scientific research but it looks like plausible explanation for "regrowing hair" issue and his regeneration capabilities generally.
Depends on the character interpretation, either his genetics and the x-gen empower his hair to grow at a superman rate to a certain length in certain areas, his hair grows at the same rate as a healthy human or his hair grows back at a superhuman rate when he suffers extreme damage to his pores and hair. For example when Wolverine shaves, his hair would take a normal amount of time to grow back but if his chin is burned heavily or his skin is cut off his healing factor would regenerate the damaged area to its original state. His current structure and appearance may be stored in his brain but that was proven incorrect when he healed from a drop of blood. So his current structure must be continually updated in code in his DNA. Look into how axolotl stem cells form and divide into a new limb anatomically correct every time, you should find the true scientific answer there.