Take a look at this picture:



Notice the shapes in the metallic coat.

Recently I am buying a lot of used CD's from the 80s and 90s. A time where the pressing art was quite plain, just words and no colour ink.
However, I can see that many of them have these non-uniform shapes, like it might be deteriorating or something. Some of them are pretty weird, with loads of weird metallic shapes.
When I extract them in EAC, all of them are perfect, don't have even a troublesome sector where the error correction triggers in EAC...

What those surface shapes mean? What is the proper name for this?
Could it be a warning that CD has hit is half-age lifetime?

A lot of them looked that way when they were new.

Aren't they Moire Fringes?

I think it's thin-film interference in the absurdly thin lacquer on the back of the disc.

Could it be a warning that CD has hit is half-age lifetime?

Since when did CDs have half-lifes?

Since when did CDs have half-lifes?

Yes. I know it is undetermined. But there also CD rot, which I believe it starts from the inner or outer edges of media.

^ It starts from the outer edge from what I saw, and they look visibly "rotting" (dark brown color), not like it is on the picture above.

You're looking at a moire pattern on the disc, caused by interference. However, the picture is of the label side, so (up to a point) the state of that side of the disc is irrelevant. If you see those patterns on the playing side (the underside with no writing) you might need to worry.

Totally normal...  IIRC, you see a rainbow spectrum when you look at the disc and the  Moire pattern when it's scanned.

Aren't they Moire Fringes?

You're looking at a moire pattern on the disc, caused by interference. However, the picture is of the label side, so (up to a point) the state of that side of the disc is irrelevant. If you see those patterns on the playing side (the underside with no writing) you might need to worry.

My thoughts exactly

Speaking of scanning...  If these patterns are look like they are printed on the CD and you're not getting a prismatic effect, or if the colors/patterns don't change when you change the lighting or viewing angle...  You probably have a homemade copy of the CD!

One funny thing is that they are only visible under incandescent/artificial light. In a room, with natural daylight, they are not visible. I will try to scan a Jarre CD here that has horrendous weird interference.

LaserDisc rot was pretty common but CDs are far less of a problem. I still have the first discs I bough April 2 1983, almost 31 years ago. I just ripped the 6 classicals from this group in the last year or two and all work with no problems. There were 11 titles available at the very beginning and I have 10 of them. Looking back I wish I had bought the 11th but I don't much care for Springsteen.

G²

Rot or not, people should definitely be concerned about the top side of the CD, as it is (especially the old ones) easy to destroy it from above.

Patterns like the OP uploaded are normal though.

What those surface shapes mean? What is the proper name for this?
Could it be a warning that CD has hit is half-age lifetime?

I would call it a less-desirable appearance.

I seemed to notice these non-circular reflections on pressings from mail order CD clubs (at least I seemed to think it since I figured there was a "catch" for getting CD's at reduced price*). At the time I thought the visible distortion-like pattern would affect playback but I really could not determine if it did.

* Your picture doesn't indicate it was from such a place since some clubs put an additional printing on the CD or back cover art, or sometimes the barcode is replaced with a catalog number. The "catch" I found was that the artists got reduced royalties on albums sold through such clubs.

This picture is from the net. I will upload a really "funny" media.