I moved in 30 years ole house 4 years ago. When I moved in, the house was newly painted. Now I found cracks on paint everywhere.

I don't worry about them 'cause it's just cosmetic. However, there's a long line of crack where ceiling meets wall in all three bedrooms upstairs.

It's like hairline crack. I don't think the gap is getting bigger, but certainly it moves along the line.

Is it normal? Or is it something that I have to fix right away?

That crack is probably nothing to worry about either. A diagonal crack at a doorway or window is frequently a sign of settling and your crack may be too. There is slight movement in your home and it could just be the humidity level changing with the seasons. Keep an eye on the cracks so you'll know if something makes a radical change but don't worry for now. Try a paintable caulk and make the smallest hole in the tube end. Strike a bead along the crack and wipe it with a wet finger or paper towel. Use white caulk and the crack should disappear for the most part.

Thank you very much. I was looking up the ceiling for 3 days. ^^ I guess I need to give some time to see the crack development and relax.

My new addition developed many cracks along the ceiling as the lumber dried out after construction. I made the patch 2 years ago and it still looks fine. Do you try to maintain a certain humidity level in your home? Cracks mean movement and drying, seldom caused by too much humidity.

You could also have truss lift as I have here. No cure found so I used a moulding between wall and ceiling.