I had pretty heavy armpit hair at age 45. I shaved every other day, but found that, even after shaving, I had a heavy "5 o'clock shadow" there. I had electrolysis on some facial hairs in the past, and assumed that it would take several sessions to make a difference with laser as well, so I signed up for a package.
The first treatment alone knocked back the coarseness of the hair such that I would have been happy, but I kept on for 7 sessions in all, until it was nearly all gone. I now have 3-4 fine brown hairs in each armpit. I can tweeze them, as shaving 3 hairs is too much bother.
The treatment did not hurt, to my perception, as I don't mind skin pain at all. There was a weird popping noise and the smell of burnt hair. Later, my armpit was sore, but after a day I was fine.
I went every 2 months for over a year, which let me notice the full progress. I epilated the regrowth in between sessions, which helps to further traumatize the hair roots, shortening their life.
My first nurse did not pay enough attention to the furthest ends of the hairy region, so that it was not until the end of the treatment period that a different nurse was more thorough and nailed them.
I was a "best case" for laser, in that the hair was black and my underarm skin was milky white. There even was a patch of white hair in one underarm area, however, which fortunately was not resistant at all to being killed by laser. I guess it was the special lotion they applied to the hair which facilitated even my unpigmented hair to be removed.
I was told that a man's back is the hardest area to treat, as the skin is thicker and thus the hair root is further from the heat generated by the laser.
I guess that armpit skin is thin, because I had a good result, and would do it again.