I'm 76 and never had an issue with unusual BO in the armpits until this year. I rarely wore deodorant and if I did I used Tom's unscented, a safer one for me because it is preservative free and aluminum free, plus free of my other tested allergens. I have Allergic Contact Dermatitis.
But it stopped working.
I like the suggested idea of baking soda and will try it, but I have found using a 50/50 of 75/25 mixture of vinegar to water splashed on the underarm for a minute in the shower then washing with a safe soap, just lard and lye or an olive oil soap (once again my contact allergies limit deoderant of soaps with a fragrance) can get me through the day or half a day. Sometimes if active or stressed by midday I am forced to wash in wash basin using this vinegar mixture, then soap and scrub with a washcloth.
If I try to just use soap the smell is not gone.
My liver was recently tested and it just was on the border of not being normal, nothing significant.