24 months is more accurate information for people who work professionally with children. (Early childhood educators, speech language pathologists, doctors, etc.) Also, fellow parents like to know exact age to compare. It's not that big a deal to use months.
This annoys people because it's more information than what is really requested. Usually, if someone asks how old a baby is, it's a polite conversation starter or topic, not a genuinely interesting subject that demands a more accurate answer.
If I measure something as the silly American I am, I'd rather be told one inch as opposed to 2.5 centimeters.
I'm not saying it's a dumb thing to do, it's just my theory.
Parents think in terms of months, because they're monitoring the baby's development. When its first word is, its first tooth, what it can eat, when it needs immunizing, important stuff like that. People who complain about this are being needlessly nitpicky.
Or, in other terms, I'm 1401 weeks old.
Or 9841 days old.
You can say a year and a half instead of 18 months.
If I measure something as the silly American I am, I'd rather be told one inch as opposed to 2.5 centimeters.
I'm not saying it's a dumb thing to do, it's just my theory.