Age is relative

Wherever you go and whoever you talk to, when the kid is around, somebody will always ask: "How old is he/she?" And this is a tricky question to answer. People, who don't have kids and haven't dealt with them much, don't realize, how much a week, even a day or two might make a difference.
There are two ends of the spectrum, as much as I have seen - people who obsess with telling everybody exactly how old their kid is, and people who round it to some closest year of half-a-year.

Non-parents don't get the first type of people, as in "Why does it matter, if the kid is a one-year-old or 13 months old?" And most of them ask just out of politeness, they don't actually want to know, that you've just stopped breastfeeding, or that kid started to use a spoon. Even if they would want to know, the development cycles and speeds of children differ so much, that they are virtually incomparable. So, why bother? It just annoys people.

On the other hand, if you tell, that kid is a one-year-old, they might expect something, that is not there yet. Or has long passed.

As a recent parent, I do understand both camps and try to take a golden mid-route - tell approximate age and go into details only if a person shows interest. As in

Them(T): "ohh, blablabla, how old is he?"
Me (M):"he's one" (been saying that for last half a year)
T: "Oh, then he should knows this and can do that"?
M: "Sure, he started doing it almost a year ago (rounding 8 months up a bit)
T: "How come? A 4 months old can't do that just yet"

And then the explanations can begin, that he's been doing it since age of 8 months and actually right now he's 16 months old.

But most of the people don't care. It's just a little kid. For us a week brings yet another weekend, while for the kid a week means a difference in whether he can pee in the pot or count or neither.