Yes, they are on any plane.
Yes. From every DS, you can access PictoChat from the main menu.
The age you stop playing ds is when you're 11.
First, you HAVE to get an attribute and action DSS card. Disregard this if you have them. Second, go to the DSS menu and select them. (There is a red box around the card you pick.) Third, get out of the start menu and press L on your Game Boy Advance or Nintendo DS.
no
Yes, allowed.
No
No
It depends on what type of multiplayer, there is multiplayer that connects to other DSs nearby, and there is the Nintendo Wi-Fi Connection which connects over the Internet. Nintendo Wi-Fi Connection needs access to the internet, local multiplayer doesn't.
Yes
No.
No.
Yes, unless the airline says something about it, you are allowed to take an mp3 player on a plane.