Depending on situation, I might assign "fault" to a monkey; with some stretch of imagination maaaybe even a pig.

Like, if it was a trained monkey, if you had mutually agreed upon rules of coexistence that were not harmful for the monkey (yes, I would be biased to evaluate this), and then the monkey for no good reason did something unusual and hurt me in a predictable (for the monkey) way.

As an example, if a trained monkey wipes its ass with my tax report, that is my fault, because I cannot blame the monkey for not recognizing the difference between paper A and paper B. If I unexpectedly try to pet a monkey in a ZOO, and it gets scared and bites me, also my fault.

If a trained monkey decides to pee on my lunch -- and the previous experience makes it obvious that this is highly unusual behavior, that it recognizes this type of food as a food and as my property, and there are no mitigating circumstances (such as having a bladder disease, or getting frightened when it innocently walked on the table) -- I would blame the monkey, because it seems to me that this behavior is recognizable as "intentionally causing harm" it its brain.

Purely pragmatically, something having moral agency seems to me to be just another way of saying "Will this thing learn to behave better if I praise/blame it." (Praise and Blame are Instrumental).

But this is, of course, just a definitional debate.

With regards to "9 month old, 18 month old, and 3 year old", and how I compare them vs Lily: I think Lily and I probably actually assign roughly the same "absolute moral agency", but I mostly interact with adults, who have more moral agency in total, and who have equivalent moral agency to me.

I think Lily recognizes 3 and 4 year olds as having roughly equivalent moral agency as herself, and one important aspect of morality is how to do coordination among equals.

(I also suspect, if Lily thought about it more and had more access to facts, she would roughly agree with Viliam about monkeys)

I found thinking about this pretty thought provoking and clarifying for me, thanks!

It's also worth exploring a bit what "fault" means. If a rock hurts you, it may not be it's fault, but you probably don't want to leave it in your path where it'll hurt you again. If a pig hurts you, you likewise need to figure out how to discourage that in the future.

Applying negative reinforcement to things with brains may be a viable option, both for things with and without moral agency. So the discussion of why this distinction matters is interesting.