[SllX]

In an indirect manner, we do mark having lived the 27th year in the following forms, we just don’t say it exactly the way you phrased it:

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.