How old is your child?

It's because parents have a basis for what 12, 18, 24, 30, etc. months are because they're used to counting in months. Non-parents who are asking how old your children are aren't asking "at what developmental stage is this little one?" They're pretty much explicitly asking "how many years has this child been out of the womb?" If you tell a non-parent "20 months" they're going to mentally go "okay, so the answer to the question I actually asked you is that the kid is almost two years old."

The burden of determining whether someone is a parent is on the person answering the question? Overall the shorter the length of time the more precise the time measurement. When someone says 4-6 weeks for delivery should that actually be 1 to 1.5 months? Or .07 to .11 years? Despite the timeframe crossing the line between measurements weeks are still appropriately precise.

No, genius. If you read what I said, the conversation went like this: You: "I don't understand the issue people have with using months for age." Me: "The issue is that when non-parents ask and get months, they don't usually understand." End of discussion. Period. There was no further qualifying argument on which way of saying a child's age is best, because I don't care how old any person's kids are by any unit of measurement.

Not sure who's arguing which poing anymore, but if we're rounding to the nearest half year... When someone asks me how old my 2 month old is, do I just respond with "oh, he's 0"