I usually use "po" to older people and people i don't know. I never once used "kuya/koyah", i usually call out to a male as "brad, bro, boss, pre" depending on the appearance, or a simple "excuse me po" to older gentlemen. For a female it's usually "Miss" or "Ma'am".
If i ever talk to someone without using "po", it could be 1 of 3 reasons. One, I consider you as one of my most closest friend. Two, if i feel we have the same vibe/atmospehere. Three, you did/said something to make me lose all respect for you.
I belive that respect should come from both ends, no matter the age. Some of the older generation seem to have no problem being disrespectful to the younger generation, and believe that they should be respected by default.