There's no way to "know". It's a best guess estimate based on average tooth wear after 1.5yo.
Theoretically, you might get 90% agreement based on the chart, but then you have to realize the chart is wrong up to 50% of the time and almost always underages the deer.
That's why it is 2.5+, 3.5+ etc.
That said, this looks like 3.5.... +