If you could give one piece of advice to the entire world what would it be?
Never follow the crowd, never get brainwashed by society. It's always important for one to have their own opinions on things. Never be afraid to stand out from the crowd. Never be afraid to walk the path that is least followed. Never be afraid of the unknown. Be brave, and dive into the unknown. Never be afraid of failures in life. Failures is just the first step toward success. Be individualistic, walk your own path and don't follow others.
Would you exist if you had no memory of your life? What constitutes the "you"?
Yes I would. Why not? Even if I have no memory of my life, at least I managed to contribute to humanity in some positive ways whilst I was alive on earth. Who knows, maybe I might have influenced someone's else life for the better. Maybe I might have saved someone from committing suicide. Maybe I might have saved someone from becoming a future serial killer. It's not the memory that makes our lives worthwhile, it's whether or not we managed to change/influence humanity for the better is what makes our lives worthwhile. As for what constitutes me, I will just sum it up in one quote, "Be the change you wanna see in this world".
Would you want immortality or eternal youth?
Definitely not eternal youth. Life on earth is like a huge test/exam, we often have to go through lots of trials and tribulations on earth, life on earth is basically a battle of willpower, who will come out the winner in the end? Will life push us down, or will we be strong enough to overcome all the challenges that life presents?
To be honest, I would much prefer existing in the afterlife. At least when I am in the afterlife I don't have to go through all those trials and tribulations that often exist in this 3rd dimensional world