Sir, you cant just post a picture of something with no description and ask how old something is without giving members any info on what you have. You dont have anything for size reference, what it's composition is, is it glass, pottery, where found (not physical address) location sich as in city in desert, in water, on land ??
Earliest would be 1960s IMO. Looks like the base to a screw top Clorox bottle.
BTW I notice that you are flooding the forum with a lot of "what is its" with most of them being in the "pieces of something" category with little description and lacking how it was found. Almost as if you are taking a stroll through a trash dump and bored.
Recommend you go through a lot of sub forums such as "Bottle and Glass" and start reading the archived posts. Many of them will answer your questions so you can limit your posts to the ones you can't figure out on your own.