But practically speaking we usually do. I always hear people refer to events in their life happening “when I was 26” and never “in the 27th year of my life”. Sure you could say the latter, but practically speaking people don’t (at least in English).
There may be something cultural that caused such a shift, like a change in how math or reading is taught (or even that it’s nearly universally taught, which changes how we think and speak because now a sizeable chunk of the population thinks in visually written words rather than sounds).
So whenever I talk time with the locals here I repeat the time back in numerical style to avoid confusion.
"The shop opens tomorrow at half ten".
"Thanks, store opens at nine thirty. See you then."
"No..."
(I had some confused conversation with a bus driver once. Bizarre experience to have so much language barrier between two EFL speakers, in English!)
But I think this also illustrates just how averse our culture is to using zero-indexing in counts: The age number absolutely is zero-indexed - a baby before before the first birthday is zero years old. But no one calls it like that, instead we drop the year count entirely and fall back to the next-largest nonzero unit, i.e. we say the baby is so-and-so-many months old. And for newborns not yet a month old, we count in weeks, etc.
I think, culturally, it's not that surprising as this method of counting is older than the entire concept of "zero". But I think it shows that there is little hope of convincing a large number of non-nerd people to start counting things with zeros.
1. On your 26th Birthday, when you say you turned 26 what it means is that you have now lived 26 years. People generally understand this, even if they are going to be spending the next year saying they are 26.
2. It is not uncommon for people to demarcate their age on their birthday in revolutions around the Sun, as a kind of meme. “I’ve now traveled around the Sun twenty-six times.” or something like that, when reflecting on their lives on their Birthday.
The colloquial usage is our legally-defined age. A shortcut for our laws to take, the age-gating ones anyway. It hasn’t replaced our cultural understanding of what the first year of our life actually was.