And of course once I drew the child and adult version, I immediately started thinking about how Rutan would look much older...

Qilins have long lifespans (often centuries), and their age can be estimated from the length of their horns, which continue to grow throughout their life. They also start to develop more scales, as despite being a hybrid of a deer and a dragon, the dragon side starts to become more prevalent in the old age as scales continue to grow and develop.