Babies as early as six months can understand basic words (whether signed or spoken), much earlier than previously thought or believed.
Unaware of this fact, I noticed that Juli understood some ASL words through her gaze pointing toward correct objects or symbols. Researching to find information on this, I came across latest research studies that can verify that baby indeed can understand from 6 months.
Understanding through behavior
For example, one day Juli and I sat together in front of each other, eating apple slices. She ate her apple pieces through the mesh feeder while I ate mine with caramel dips.
Juli appeared to imitate me by dipping her mesh feeder in the plate repeatedly until my apple slices ran out. (see an example in 2nd video below).
Juli dropped her mesh feeder behind herself every time I signed done/finished.
When the apple slices and the caramel dip were empty, I explained (translated as) "Done, we were done eating apple". She then dropped her mesh feeder. I picked up and signed "more".
Juli resumed eating for a bit while. Then I informed her that we were done. She eventually dropped her feeder again.
New research indicates that infants as young as 6 months can understand the meaning of many spoken words. They at this age don't say nor point, but actually they put together the things in the world along with the words. [Ref]
Understanding through gaze pointing
A month later (m8w1), Juli demonstrated ASL receptive skill and understanding. For example, sitting near the jumper under the sofa, I asked Juli if she wanted to jump, want jump?. Juli's gaze turned to the jumper under the sofa. I noticed the first time and observed this again the next times. Same results.
Gazing is an advantage that helped me recognize Juli's comprehension, when word or finger-pointing didn't emerge yet. Baby does not finger point until around 9-12 months other than reaching and grasping.
Even Elliott the Cat understands or at least it seems.
Juli's furry friend Elliott the housecat had never met a baby before. The first time the cat met Juli, she trusted her with all her heart.
But, as Juli grew and grasped things. Elliott took a precaution. Once Juli grasped Elliott's tail and furry skin, Elliott maintained a cautious distance.
Juli walked with my support toward the sitting cat. Elliott took a step above on the stairs. Her animal instinct took over. As Juli grabbed high, Elliott snapped with her paw near Juli's hand as if to give a warning.
I picked up Juli. I gently looked into Elliott's eyes and talked to him in ASL, #nono, you-know ix[baby] just baby (translated as), "No, no. You know, she's just a baby."
From there on, Elliott joined us comfortably, lying down right next to Juli even when Juli touched him. Elliott was aware that I made sure that Elliott wouldn't get hurt.
It's not like Elliott understood ASL words. But, he appeared to understood the tones, moods, and the contexts surrounding words.