I think theres a gradual change.
To me you move from weeks to months at around 13 weeks/3 months.
Earlier than that, months is too vague.
A "one month old" could be a 4 week old newborn or an 8 week old small baby who has had their first jabs.
Similarly with months and years.
I always switch from one to the other at around 2.
So a 13 month old. A 15 month old. Even a 22 month old.
But once you're at 2 years you can give enough information with either 2 or 2 and a half.
Once you get to 3 just give the numerical age, unless they are very soon to have a birthday. In which case you say "nearly 4" (or whatever).
Not that it matters much, or that it's "precious" to be more precise.
But to my mind those are the things that make the most sense to the most people. 30 months leaves people doing sums in their head.
Which might be good for them, but it does slightly obscure meaning :o
I think the 32+2 thing, on a parenting board where lots and lots of newcomers are pregnant with their first child and spend most of their time on pregnancy threads talking about it, is totally fine and we we should leave people alone about doing it.