Is it safe to apply a screen protector(scratch guard ) on MacBook Air ?

Do not use a screen protector.

From: Using a camera cover, palm rest, or keyboard cover on a Mac notebook - Apple Support

"Leaving any material on your display, keyboard, or palm rest might

interfere with the display when it's closed and cause damage to your

display."

How old a computer? Once they reach about 5 years old they reach a point where Apple won't repair them anymore should the need arise. Until then, should you need to get it repaired, I wouldn't do anything to the computer that could be deemed by Apple as having modified the device. That definitely goes if the device is still under warranty.

I agree with BobTheFisherman. If it were an iPad, I may consider it because those have a touch screen. A MacBook's display is only visually interactive and normally shouldn't be touched.

The tolerances between the display glass and the keyboard are very small. Putting a screen protector on the display glass may put unexpected pressure on it if it contacts the keyboard when the display is closed.

Also, do NOT install any type of keyboard membrane "cover" or anything on the top case for protective purposes as this may have the opposite effect of causing display glass damage.

Given the number of complaints we see here about screen "protectors" doing serious and permanent damage to the coating Apple uses on displays, evidence says that is a bad plan. I just retired a Macbook Pro I bought in the summer of 2013 and, after years of travel and hard use with no "protectors," the display cover is perfect. Tougher than people think.

The other issue is that, in newer Mac laptop computers, the display is part of one sealed assembly. Before that you could replace a damaged screen cover for very little cost. Now, damage to any component of the display assembly requires an entire new assembly. We cant quote costs but, based on people posting here who have suffered, parts + labor costs for newer Mac laptops seem to start around US$500 and go up from there.

When I bought my Retina MacBook Pro, I also bought a thin microfiber cloth, which I put onto the keyboard before closing the display.

I did not want to attach something adhesive to the display.