How can we see stars if they are hundred years of light far from the Earth? So the right question is, is the light years counted from that second when you look up to the sky? If that so, how can we see the stars from the telescope (which I think it starts when you put your eyes on it, same as when you look up to the sky), that they say it's 5000 of light years from earth? If you say these lights are already started 5000 years of light ago, then there should be so many changes happened to these galaxies or stars in that timeline. Thus, for example, there should be no way to know, if an earth-like extrasolar planet is still earth-like.
You are right, there is no way to know, what is the current state of these objects "now" - if you think on the Newtonian mechanics.

For example, the 1054 supernova, what created the Crabs Nebula, happened actually not in 1054, but around in 6000 B.C. It is because it is $\approx$ 7000 ly away from us.

In the Special Relativity, "now" is relative. You can't say, that the 1054 Supernova happened in fact 7000 year ago: if you would see the whole story (1. the star explodes 2. the astronomers of a planet far, far away, some millenia later, detect it) from a spaceship going, for example, with half the light speed, you would get totally different numbers. There is no "objective now" in the special relativity.

For us, "now" is what we see in the telescopes. For us, the crab nebula explosion happened in 1054. We have no way to visit a star, for example, 50 light year away, now - we would need at least 50 years to visit it. (It is the theoreitcal limit - in practice, there is no technology to send anything even to the nearest star, 4.3 light years away. If there would be, it would likely require at least centuries to get there.)

However, the overwhelming majority of the stellar evolution processes is much longer. Most stars exists between 10million and 10billion years long. Of course we have no way to watch a star billions years long with the telescopes. However, we can see billions of stars, typically some decade (centuries) long, and even this is enough to understand a lot from them.

We have a strong reason to not expect a huge difference from the "current" state of most stars/extraplanets from what we can see today on them - simply because such events (change of planetary orbits, average lifetime of most stars) last million times longer.