Aging is a step function not a smooth line
I don't think young people in their teens and early 20s realize that aging for a lot of people happens in these distinct bursts. You can look pretty much the same for years and then within 6 months all of a sudden you have more lines near your eyes and mouth, you are a little more jowl-y or a bit more angular.
For me, until I was 35, I regularly had people act surprised that I was "so old." I looked basically the same for about 10 years. But in the last year or so I definitely see more obvious signs of aging.