I can remember life in general to age 2, and a few scattered memories from apx age 18mo onward. Some adults remember NOTHING prior to kindergarten, some adults only have hazy memories until their teen years.
There's a lot of research going into memory, currently, but not a lot of findings. My personal opinion is that it's because people's childhoods AND their personalities are so different in ADDITION to brain structure.
Anecdotally: abused kids TEND to not remember most of their childhood as adults ((their memories TEND to start when the first gain control over their lives OR their living situation changes (fostercare, living with non-abusive relatives is an example; but it's only *probable* that their memories will kick in when that living situtaion changes frequently... fosterkids are actually fairly notorious for "listing" all their families they've lived with. Some will write on the bottom of a shoe, others on their leg or arm, others on a baseball cap.)) But some abused kids have vivid memories of the childhood. DITTO some NONabused kids have sketchy memories of their childhood.
Anecdotally: Kids who move frequently TEND to have earlier memories than kids who stay in the same home from birth - several years later. Military kids (moving every 2 years) tend to have memories starting at date of first move after 1yo. Why this doesn't work with kids who don't changes states/countries is unknown (but easy to theorize... air, light, smells, water, foods, sounds, weather, etc are all geographically QUITE different from moving in the same city). But not all kids who move frequently (even from states and countries) have early memories.
At this point NO ONE KNOWS why some kids and adults have early memories and some don't. Ongoing research. Again... a lot is easy to theorize (In this CULTURE most kids remember starting at K, because it's so vastly different from their previous lives in the SAHM generation... but what about kids in daycare and preschool from the Double Income generation? Is it because of the difference OR because a brain maturity level is met by the *majority* of the species by age 5?) but no one really knows as YET.
I remember back as far as 18mo... but I only nursed to 1yo. Don't remember it. I also only vaguely remember diapers... but I was totally potty trained before 18mo (none of my sibs were, though).