I just try not to think in terms of age, but watch my child instead. I don't have a silver bullet about how to do it! Maybe the closest I can come is that I try (not always totally successfully) to think in terms of process more than results, i.e. to think about what the next thing DS needs to work on is to get to the next level of responsible-ness, regardless of where he is in absolute terms.
However, one still has to choose what to take as a teachable moment and what to let go. It is tricky, because sometimes age-typical isn't the right aim. E.g., if he's going to talk with an adult vocabulary, it's incongruous if he's still interrupting like a 6yo, and it becomes all the more important that he doesn't.
