Depending on the child, I believe 5-6 yrs is appropriate for lessons on consequences and it's relationship to responsibility. I'm sure one of the Pros here can cite materials, but my advice comes from personal experience.
This is generally an age where you can have a reasonable conversation. They can remember to bring papers home from school (or daycare or even home-care). They can follow the basic "rules of the road" at school, can likely recite to you why they follow the rules and then tell you want will happen if they don't. In other words, they are familiar with consequences and I think a child this age is ready to link responsibility to those consequences.
In my experience, where most parent's fall down is with clarity and consistency. For my part, when talking about their actions, I will usually point out what was done and what happened. I don't tolerate "but i was just" kind of justifications. [tangent: i avoid using the word 'excuses' or derivatives as I believe that it has an inherently negative connotation] When they start coming, I tend to go into 'slam' mode...
No, Finn, it was not an accident. You were the one that was playing rough, You threw the brick (megablock) at Marceline and You were the one that made her cry. That is on You, not her.
This 'slams' the door on further 'justifications'. It is usually followed by whatever the consequences need to be. So in the above situation, I'd say my piece, let it soak in for a couple seconds (basically waiting for reaction) and then go on to consequence phase.
This says clearly where the responsibility lies in a very plain and straight-forward manner without dwelling. Conversely, I apply the same technique when talking about positive actions and handing out praise. Balanced reinforcement worked/works for me as a method of teaching responsibility.
However, whatever your approach is, you must be consistent. There may actually be extenuating circumstances or a truly accurate justification, but you cannot change your approach depending on the situation and then expect the child to learn to own their role in particular events. Be prepared to have that conversation when it needs to be had, whether you're at the store, at home, on the subway or in front of relatives.
It's a hard lesson, and there may be tears involved (yours and theirs). But -- go you -- you have identified the need for an essential lesson, early. Whatever your approach, at this early stage I think you will be able to work on it effectively.