When my son was a little bit younger than that, he freaked me out in a similar fashion. He was being a little fidgety, and I was trying to get him to go to sleep. At the time we lived in a four level home in Wiesbaden, Germany. It had a lot of stairs, but not much room to pace, so I carried him up and down the stairs. As we were going down the stairs I heard him speaking, very softly. Since his head was on my shoulder I could just barely make out what he was saying. As I stepped down on each step he would say “Ein” step “Zwei” step “Drei” step “Vier”. etc.*
This was especially weird as we didn’t speak German at home. I can only guess he got it from “die Teletubbies” or “Sesamestrasse”.
I didn’t notice how far he got. Probably not to fifteen.
One I particularly remember was when our daughter was about 15 months old and talking up a storm. Her pediatrician noticed her saying the name of a color and expressed some suprise. Oh yes, we said, she knows them all. The doctor didn’t believe us until she tried it out herself with a box of crayons. Apparently, knowing at least one color name is a developmental milestone–for 3 or 4 year olds.