Not certain if there is a practical, factual answer to this (Mods kindly move if there isn’t :)) but I’ve always wondered why many parents count this way. A few I’ve met have persisted in this habit well into the third year. Even the children themselves can tell you their own age in years by that point. Obviously during the first year it is relevant, but after that I can’t see any good reason for such cumbersome counting.
Actual conversation I had once:
Me: So, how old is she now?
Mother: Almost 40 months.
Kid: No, mom, I’m THREE!
I think it’s to pad the age.
Similar to children saying that they’re 8 and a half rather than just 8.
You kids don’t have much age to speak of, so any thing that makes it more impressive is a go.
I had thought it was because it’s convenient with respect to child development benchmarks. So instead of saying “most toddlers do X/acquire skill Y/etc. by one and one-quarter years,” you can say “most toddlers … by 15 months.”
It’s because development happens so rapidly during the first few years that to count age in whole years is meaningless. Because pretty much all developmental steps happen within a range of time, to say that a child was a year old when he learned to walk isn’t very helpful or very accurate. A child who learns to walk at twenty months is still a year old, but that may reflect a developmental problem that should be investigated.
So it’s simply a convention that parents use because it’s more useful and more accurate than reckoning time in whole years.
So when speaking casually about age, and not any sort of milestone, this is just a habit then?
It can be useful. I would often say my daughter was a certain age and people would be surprised at what she could do. They thought she was a genius, doing things that most kids wouldn’t do for another few months and I would have to clarify that she was actually X months old, which was closer to expectations.
As others have mentioned, it’s just more accurate to reference a young child’s age in months, even in casual conversation. I always used months rather than years when my kids were small.
Look at it this way: referencing your 6mo-18mo old child’s age in years would be like referencing your 5-15-year old child’s age in decades.
I’m not trying to be argumentative here, but how is 18 months any more accurate than A year and a half ? I’ll give you that until two it could make sense, but why would it continue after that?
I agree with this. Once my children where over a year I gave their ages in years (and months if needed for clarity) milestones be damned. Of course my oldest child is now 224 months old so I may just be too old-fashioned to understand the significance. 
Note that neither of my daughters just showed up at the doorstep at the age of a year and a half. For awhile they were thirteen months old, then fourteen months, then fifteen months…
So when one of them would hit their eighteenth month there didn’t seem to be any particular reason to switch to “a year and a half”.
And I will admit that we in particular referred to their age for awhile as “she’s in her terrible twos” 
Pediatricians’ offices tend to refer to kids under age 2 by months, possibly because until that point, most practices still want you to come in at least twice a year for well-child checkups. So you’ll schedule your 15-month checkup, 18-month checkup, etc. Typically at age 2, you start coming in once a year. So maybe parents are picking it up from the doctor’s office as well.
I did this until the sprog was three or so. Once he turned three, he jumped from three to four, not from 36 to 48 months.
To answer your question, it’s just a convention that most parents pick up from pediatrician’s offices and the children’s paraphernalia industry. It means more in those two contexts than it does to anyone else.
I think people continue using only months up until the age of 2 just because it’s easier to say 17 months than 1 year and 5 months. Around here I’ve noticed that once the child turns 2 parents will just say, “She turned 2 in August” or “He’ll be 3 in February” so the age is better represented. After the age of 5 you just get halves or quarters, usually from the kid.
It kind of matches up to one of the more popular developmental assessment charts.
Up until a year, I counted my kid in months. Between 1 yr and 2, I mostly did “1 year and x months” or “birthday is next month”. I love my kid, but honestly, most people who are asking such a question don’t really care if she’s 20 months and I say a year and a half.
I overheard some parents the other day talking about how their kids were 27 months or 25 and even though the math is very easy, I still had to pause a minute to work it out. “He turned 2 last month” is going to be more than sufficient 99% of the time in casual conversations.
I think it’s especially relevant up to 18 months, and fairly useful/common up to 24. I know that once my daughter reached her second birthday, I switched to years/half-years. Clothing sizes for children, IIRC, do this as well–you can buy clothing for 12 mos., 18 mos., 24 mos., but that’s when they also switch to “toddler sizes”, like 2T (which is essentially the same as 24 mos.), 3T, etc.
A couple of conversations I had:
(While holding my 7-month-old daughter at church function)
Church Lady: How old is she?
Me: Zero.
(My mom is introducting me [as an adult] to a friend of hers)
Mom: And this is my baby…
Me: I’m 476 months!
I think the factual aspects of this have pretty much been addressed. Moving to IMHO from GQ.
Colibri
General Questions Moderator
I usually use a unit until I reach two of the next bigger unit.
So, age in days until they are two weeks old.
Age in weeks until they are two months old.
Age in months until they are two years old.
I find that this works reasonably well for differentiating kids at all ages, in terms of the answer having something meaningful to do with their developmental stage.
Sweet. Next July I’ll be 300 Months old!
FOR SPARTAAAA! That makes looking forwards to my birthday inherently more awesome.