Your character doesn't have to be an adult - you can begin collecting the stamps when your tamagotchi is young. This is because the symbols do not change depending on your character; they are the same for each new generation. While your character doesn't need to be an adult, it might become an adult while you wait to collect happy symbols as some symbols depend on the weather outside, or require you to buy expensive items (eg. telescope) from the shop.
Binary has a great TMGC+C guide here on Tamatalk where all of the happy symbols are explained. Here's the link to the happy signs part of the guide: http://www.tamatalk....e/#entry2595344
Edit: your character can begin collecting happy signs at any age
Edited by Counting Bodies Like Sheep, 08 December 2013 - 04:37 AM.