I get skin flakes flying off my chin, especially when my beard starts to grow. this has been a problem for years now (31 yo m). what is the likely problem here?

Shaving is a very good way to eliminate skin flakes; growing a beard eliminates this method of exfoliation. Scrubbing with a washcloth won't eliminate the problem; you may consider a mild keratolytic like an acne topical preparation containing salicylic acid. Be careful as too-frequent use can dry the skin and worsen this problem.

Probably seborrheic dermatitis. Try an OTC anti fungal cream twice a day -- if that doesn't work, it could be something else and you'll need to see a dermatologist.