What do "sin" and "death" mean in Romans 5:12? Romans 5:12 NASB
  Therefore, just as through one man sin entered into the world, and death through sin, and so death spread to all men, because all sinned—


In my country's language, the word "sinned" is translated as an act against God's will. And the word "death" means spiritually dead.

So my interpretation is that someone— either a very young child age of 3 or a teenager to adult— has to do something against God's will first, then this person is spiritually dead. So (for example) if a toddler has not done something which is against God's will yet, this toddler is still spiritually alive.

My questions:


In the English language world, does the word "sinned" in that verse also mean an act against God's will or something else?
In the English language world, what does the word "death" mean in that verse? Does it mean that the person is not breathing anymore, or that they are spiritually dead?
Your first question:


  In the English language world, does the word "sinned" in that verse also mean an act against God's will or something else?


My answer to this would be that the Oxford dictionary of English states that the word "sin {verb; sins, sinning, sinned}" means: 


  "an immoral act considered to be a transgression against divine law {an act regarded as a serious or regrettable fault, offence, or omission}.


Your second question:


  In the English language world, what does the word "death" mean in that verse? Does it mean that the person is not breathing anymore, or that they are spiritually dead?


My answer to your second point is that the word "death" found in Romans 5:12 can be referenced as physical and spiritual death simultaneously {see below my understanding of the text where the emphasis is mine}.  


  Therefore, just as through one man {Adam} sin entered into the world {because He disobeyed God in Eden thus cursing the whole creation}, and death through sin {because the creation is no longer perfect, but fallen due to sin; hence why we die}, and so death spread to all men, because all sinned {because we are all descendants of Adam it is in our nature to sin in thought, word and deed}.