However, basically the longer hair follicle, found on the scalp, has a longer growth period than pubic or armpit hair follicles. So the hair on the scalp is shed very slowly compared to the other regions. Compare this:
Scalp hair is about 1 cm per month or 0.34 to 0.36 mm per day throughout life. Meanwhile, eyelash or brow growth at 0.16 mm per day in average. Beard hair at 0.38 mm/day. Once again, it depends on hormone. I shave my beard every three days, while my friend does it every day.
What about pubic and axillary hair? They grow almost as fast as scalp hair at 15, but slow with age and have a much faster follicle turn over time. The genes control it all automatically. That’s why your armpit hair can’t reach your stomach, even if you never shaved it since you were born.