How old was people at the time of our first memory--a memory we are certain of, rather than one that seems like a memory because others have told us...

How old was people at the time of our first memory--a memory we are certain of, rather than one that seems like a memory because others have told us about the event so many times. How are we sure it's a memory? Why do we can't all remember back to the moment of birth, or even before?