Thank you for providing its precise size measurement. It seems to be an ornate Lady's Brooch, because its back shows the remnants of the typical brooch/pin "attachment" provision, a pin-clasp for attaching it to clothing. The clasp's pin is missing, but the pin's tip would be held under the prong on the opposite edge of the brooch's back.
The tiny prongs around the central hole on the brooch's front probably held a "gemstone" or perhaps an emblem, which is missing.
You asked "How old?"
Unfortunately, there is no way to date this form of brooch with high accuracy. It is made of very thin machine-stamped sheetbrass, which dates it from approximately the mid-1800s all the way into to the modern era. But based on certain peculiarities on the attachment-clasp's form, I would think it is latter-1800s to early 20th-Century. In particular, the swirly design with
cutouts was popular in the latter Victorian era ladies' brooches. Our fellow What-Is-It forum member Breezie is far more knowledgeable about antique ladies' brooches than I am. She might contradict nearly everything I've told you.

Try sending her a Private Message about it.