I am very old, but I won't disappear soon.
Every day, I go through you, but you cannot feel me.
Some says that I am fictional, but I am visible.
Only happy few really saw me.
I move, following the one path again and again, faster than sound.
Who -or what- I am?
Hint 1
I am very fast and very slow at the same time, but I have always the same shape
Hint 2
You can see my fellow on the moon
Hint 3
a famous character took my name
You are
The line separating day from night
I am very old, but I won't disappear soon.
Indeed. You're as old as the rotation of the Earth itself.
Every day, I go through you, but you cannot feel me.
Yup - the dividing line passes right through us, but cannot be felt.
Some says that I am fictional, but I am visible.
The difference between night and day is an oft-used metaphor, but it is indeed a real thing and can be seen.
Only happy few really saw me.
... Of course it is really only visible from space, and only a very happy few have ever gotten to witness it.
I move, following the one path again and again, faster than sound.
The line moves around the Earth once a day, always following the same path, and travels well in excess of the speed of sound.
Hint 1
I am very fast and very slow at the same time, but I have always the same shape
Very fast, in that it travels over 1000 mph at the equator, but very slow in that it takes all day to finish its trek. And always the same shape - a line, moving in an orbit around the Earth.
Hint 2
You can see my fellow on the moon
You can also see the dividing line between the lit and dark areas of the moon, as it changes from full moon to new moon and back each month.
Hint 3
a famous character took my name
The dividing line between night and day has a name.
It is called the terminator.
There's also, of course, a very famous film (and its sequels) about The Terminator.
Is it
Light
I am very old, but I won't disappear soon.
Stars are very old and take a long time to die, so light will be given off for a long time.
Every day, I go through you, but you cannot fell me.
Light shines and hits you but you can't destroy it.
Some says that I am fictional, but I am visible.
The light at the end of the tunnel when you die, some people don't believe in it. Light is visible in a way because it lets us see.
Only happy few really saw me.
If you see the light, you are enlightened.
I move, following the one path again and again, faster than sound.
Light goes in a straight line (until it is reflected, but still in straight lines) and it travels faster than the speed of sound.
Hint 1:
Light travels at different wavelengths.
Hint 2:
You can only see the light side of the moon.
Hint 3:
Light Yagami from Death Note
I think this is wrong, but...
Earth's orbit
I am very old, but I won't disappear soon.
4 billion years or so with, I think, a few billion to go
Every day, I go through you, but you cannot feel me.
We're all along for the ride but fairly oblivious to the actual motion
Some says that I am fictional, but I am visible.
Some still say the earth is flat. Probably some still say it's the center of the universe, so not orbiting anything.
Only happy few really saw me.
Only a few have been up in space and seen it from a different perspective
I move, following the one path again and again, faster than sound.
Who -or what- I am?
The earth is moving, I'm assuming, faster than the speed of sound relative to ... whatever
Hint 1
I am very fast and very slow at the same time, but I have always the same shape
The earth is moving very fast around the sun, but from our perspective it's very slow - it takes a whole year - but the path is roughly always the same shape.
Hint 2
You can see my fellow on the moon
The moon has its own orbit around the earth
Hint 3
a famous character took my name
I'm not sure about this one. I'm just going to say Orbit Gum.
I think it's
Gravity
I am very old, but I won't disappear soon.
Gravity has existed since the dawn of the universe, and it will continue to until the end of it.
Every day, I go through you, but you cannot feel me.
Gravity pulls us towards the Earth.
Some says that I am fictional, but I am visible.
The gravity is invisible, but it is observable.
Only happy few really saw me.
Discovery of gravitational waves.
I move, following the one path again and again, faster than sound.
Gravity keeps pulling things towards the center of mass, so it keeps following the same path, over and over again. Gravitational waves travel at the speed of light, so they are way faster than sound.
Hint 1:
Not really sure.
Hint 2:
Moon orbits the Earth thanks to gravity.
Hint 3:
Gravity from Marvel
Is it
RAINBOW
I am very old, but I won't disappear soon.
sometimes rainbow stay long.
Every day, I go through you, but you cannot fell me.
can't feel rainbow
Some says that I am fictional, but I am visible.
7 colours
Only happy few really saw me.
few can see it.
I move, following the one path again and again, faster than sound.
Light travels faster than sound.
Neutrinos
As:
Neutrinos 'old as time'- they were released in the big bang. They pass through 'everything', to the point detection is nearly impossible. As such, the few people who have managed to detect a neutrino burst are extremely happy with their result. Also Neutrinos travel 'near the speed of light'.
Taking into account @user1566694 and @PaigeMeinke answers were close to the real answer, you probably are:
A day
I am very old, but I won't disappear soon.
The concept of day comes from ancient ages. And probably will not disappear.
Every day, I go through you, but you cannot feel me.
Every day you go through a whole day, obviously.
Some says that I am fictional, but I am visible.
A day is a concept that human invented, so can be fictional and it is invisible.
Only happy few really saw me.
People in space are able to see a whole rotation of Earth, that is a day.
I move, following the one path again and again, faster than sound.
A day is doing the same path (Earth rotation) again and again faster than sound.
Last answers are nearly the same as @user1566694.
Hint 1:
A day can be long or short depending on if you enjoy your time or not, but always lasts the same.
Hint 2:
When you see the moon is at night, the night can be the fellow of the day.
Hint 3:
Lots of characters are called 'Day'.
It is perhaps a
Halo,
As,
I am very old, but I won't disappear soon.
the halos are there since time immemorial and also going to stay there!
Every day, I go through you, but you cannot fell me.
A halo is a special formation of light which can be visible / invisible and we cannot avoid it (as human body is permeable to some form of rays in "spectrum")
Some says that I am fictional, but I am visible.
Halos around divine / famous characters are fictional, but a halo around a celestial object is visible (for example: the moon)
Only happy few really saw me.
If you are lucky enough, you can enjoy a halo during a day / night.
I move, following the one path again and again, faster than sound.
Not sure about this, but since it is formed by light, it can be faster than light and since path is circular, it appears to be travelling in the same path again and again, though in reality, it is NOT
