Looking for Jesus in the details.

Now Jesus himself was about thirty years old when he began his ministry. Luke 3:23

One curious aspect about the Bible, and religious texts in general is how we hang our hats on so much specific dialogue as if biblical characters (if they even existed) were being shadowed by stenographers. This particular sentence illustrates the point. How does the Luke writer know exactly what Jesus spoke but not know exactly how old he was? This may seem like an easily explained minor detail, but if you consider how often small phrases are utilized to justify complete positions on individual issues or even full dogmatic policy; this matter because much more significant.