Well for number one i dont think there is an age limit because i believe there was footage of one of the avatars being known from birth, i cant remember the specifics and may be wrong. Avatars also like certain toys and are suspected im sure long before the age of 16.

If they are the avatar, i would think that there connection to the spirit world, where all their past lives preside, will be made when they find out they are the avatar. Because it always seems like Aang was looking for a way to talk to the past spirits, if you dont know they are there you cant talk to them... unless you are crazy...

The Air Nomads sensed that there was trouble brewing ahead, and they felt that a war was upon. They must have realized that if they told Aang he was the Avatar and then sent him on his merry way to stop the war he would probably not be able to handle both the revelation of who he is and the actual conflict. I think that the Air Nomads decided to tell him so that Aang would already handle the whole I'm the Avatar deal, and so that they did not have to lie to him when they worked his arse off with high level bending maneuvers in order to better prepare him.

1. I think that the power of the Avatar doesn't fully manifest until the person accepts who they are, just look at Zuko's bending. It shut off because he had no real drive; so maybe the Avatar State only functions properly when the Avatar accepts who they are, and the power they hold.

2. The fact that they kept it a secret was to help the Avatar live as normal a life as possible. Yang Chen explained it in Escape from the Spirit World, if an Avatar was always of who they were and if they had not made any friendships with people they would be more inclined to ignore the troubles of the world. So I believe that they do not meet any of their past lives until they realize who they are.

Your first thing about getting to the avatarstate and having to be at one with your self and all is correct, as it was pretty much said in the Guru. Aang had to open his chukras (spelling) and by doing so had to accept everything and stuff like that.

~Pago

Um, Shio, in the episode 'The Boy in the Iceberg', Aang was stuck in the iceberg because he ran away from the Southern Air Temple. I know it was partly because they were going to take Monk Gyatso from him, but also because he didnt want to be the Avatar. To me, he doesnt seem to accept his destiny. In the episode immidiantly following, 'The Avatar Returns', Aang and Zuko get into a quarlle. Zuko throws Aang off the ship. He comes back up in the Avatar State. If you have to accept that your the avatar, how could Aang have done it?

You throw a good point there, but i think it was less of accepting he was avatar and more of he just didnt want to be the avatar. I think that he knew he would have to be the avatar, but his reluctance only came from the fact he didnt think he could do it, insecurity.

I think it's more of an issue of mortal danger than having a "pretty bad day." Aang, trying to run away from his problems, entered the Avatar State when he and Appa were in danger of drowning. In "The Avatar Returns" he uses the Avatar State again to save himself from drowning after being hit overboard by Zuko.

The kind of emotional pain it takes to enter the Avatar State is pretty extreme, like realizing your mentor/father figure died fighting firebenders, believing the girl you love is dying, and having your best friend kidnapped and sold to a circus. Considering that the state of the world before Aang was relatively peaceful, I'm willing to bet that circumstances didn't pop up that would cause the Avatar State before the Avatar was notified. Not too many life threatening situations, and none of the past four Avatars before Aang showed any signs of extreme emotional stress until after taking on the role.

I think it would pose a problem if an Avatar, before knowing that they are, had such an extreme situation.

I think your right, because im reminded when Aang visited the earth village and they tried to force him into the avatar state, which they did by threatening him and his friends. So i agree its a sense of danger and overwhelming emotion.