I was about 18 months old -- I might have to check with my mother again. She doesn't believe I remember it, but can't explain it any other way.
My grandfather was building their new house (so it can be dated) -- there was only a foundation at that point, but there were open backed stairs without any handrails. I went down to retrieve a ball, but had trouble getting back up the stairs -- because of the open backs on the stairs.
Took me many years to overcome my fear of open backed stairs -- and I still (at age 60) have a fear of heights. (Can't get past the third rung of a ladder.) Probably all dates back to this experience.
I have some other very clear memories at age 3 and 4, dated by school (at age 3, a crafts class at Pershing School; 4-year-old kindergarten the next year) and where I was living (47th St. in West Milwaukee, Wisconsin). Wait a minute, I remember a bit about the previous place we lived, too, so that would be somewhere in between the 18 months and 3 years old.