#1
I was just wondering if this goes for everyone

Take for example Cat Stevens, Neil Young, or James Taylor (folk singers). They've been singing since their 20s, and still are singing, but they sound the same to me. Obviously not exactly the same like back in the day but not that much of a difference in voice change.

Now take artists like Bobby Bland, Ray Charles, Van Morrison (all soul singers),

When they started out, their singing sounded different compared to how they sang when they were in their mid 20s to 30s. Bobby's voice developed more and more as well as Morrison's and Ray's. So you could definitely hear a difference if you hear how they sounded when they were younger, compared to their mid 20s, till now. (except Ray, since he passed away).

So, did ^these guys' voices develop more and changed drastically as they aged because they sang differently than a folk singer?
Folk is usually soft on vocals and not as strong as a soul singer's.
#2
Every singer I know has. I remember that I was listening to Heaven and Hell's album "The devil you know" (which was with dio) and thought he had epic vocals, but if you listen to Holy Diver by dio you can tell that his voice has changed.

James hetfield nuked his vocal chords.

Dave mustaine's voice isn't as dynamic as it use to be, but it never really was amazing.
#3
Yes, you will change as your voice matures and gets older. However, most of the time, your voice will still be very recognizable as YOU no matter what age you are. For example, David Gilmour's voice changed a bit over the years, but everytime I hear him sing or talk, I instantly know it's him.
#4
Yeah, that's very much true, usually. Take Phil Anselmo, for example. In Power Metal he had a squeakier voice than Dave did at times. Then again, he was doing Judas Priest and such with the high screams. His voice then is much different from what it is now: a battered half-mess, half-miracle brought down from screaming about half an octave in speaking range.