Nemo age 9: In chess, it's called Zugzwang... when the only viable move.
Nemo Nobody aged 118: ...is not to move.
Nemo Nobody aged 118: It should be written on every schoolroom blackboard: life is a playground or nothing.
Young journalist: I'm sorry, I don't understand. Did you stay with your father or go with your mother?
Man in black 2: This is weird. His shoes are different sizes.
Man in black 1: Maybe he shrunk? Happens when you get older, you shrink.
Man in black 2: No one shrinks, that's rubbish. You got the wrong bloke, that's all.
Man in black 1: Astronauts shrink two inches when they come back to Earth.
Man in black 2: You think this fella is an astronaut?
Nemo's Father: You look like my son.
Nemo age 16: I am your son, dad.
Nemo's Father: My son is taller than you.
Answer: This film is mostly open up for interpretation but I understand it that young Nemo thought up older Nemo and it actually happened, maybe in an alternate universe. Don't forget that Nemo could remember the future so most of what he imagines is probably going to happen. Unfortunately he can't predict choice so he doesn't know which life happens so all of them happen at once and they don't happen as well because he chose to make no choice (Zugzwang). Hope that explained it.