A friend of ours was approaching her 40th birthday. But she only admitted to having a "round" birthday, and even forbade her kids to talk about mommys age, even though we made a big production to learn her age with trick questions and all. And then we told her that if she keeps mum about her age, we'll just guesstimate it.
That's how she got sent loads of cards for her 50th birthday. She was a bit pissed when we arrived for the party. And then we handed her the real cards for her 40th.
Migrants from war torn countries unfortunately. In some communities there's a lot of people with birthdays on Jan 1st, because they don't know their birthdates. Quite common for Sudanese refugees (places with child soldiers or where family/parent based killing happened).
People still don't record accurate birthdays here. A registrar goes to a village once every 3-4 months and all the kids born within the previous few months get the same birthday (if registered at all).
And this is a new thing. People older than that from villages have no clue how old they are. Just guess stuff.
There comes an age where you stop counting. That's when people start referring to age by "born in year" and not as a number. Never understood my parents why they did that when I was a kid or teenager. I do it myself now, think it started with 35 or so. Hope I'm not a vampire....
My grandmother often gave us birthday cards with the age we had before. At some point she switched to generic cards with a number on it but a question mark would have done too