You have to use a masculine adjective, because 'człowiek' (and hence 'ludzie') is a masculine noun. The noun itself, however, can refer to people of either gender.
Well... it would sure show your point and prove that you don't speak Polish, as your sentence means something like "I not in Polish" ;)
What you meant is: "(Ja) nie mówię po polsku" :)
The older people speak polish
if you have elderly then older has a case
elderly can be without "people" as this would be a taken
"Starze" as "old" not exist - "starzy" is correct. "Staże" sounds the same and means "practices".