What drawn images mean, however, is that a character in a single manga or graphic novel can vary in appearance. Marriage counselor Serizawa from The Love Guide by Ayano Yamane looks his assigned age in the first panel (20-ish), somewhat younger in the second. Out of curiosity, I examined multiple panels closely to figure out the difference.
Close examination is how cartoonists, mangaka, graphic artists, and anime artists work. With Gollum, for example, from day 1, Jackson et al. used minute details plus Andy Serkis's facial expressions to give Gollum a personality. (Of course, this type of thing can create the uncanny valley.)
In the case of Serizawa, what makes a person older? How do we humans age?
1. Longer chin
2. More defined nose
3. Not quite so large eyes
4. Wider mouth proportionally
For example, here is Harry Potter younger and Harry Potter older over the first two films. In the Chronicles of Narnia, Skandar Keynes on occasion appears to be aging on screen (growing teenagers must cause havoc with director continuity).
I took a young image of Serizawa and aged it. Below is the result.