Can we set an age of criminal responsibility that will effectively provide for the right punishment?

Two young men both separately carry out the crime of murder.  Both use a gun, both do it intentionally, and neither one acts in self defense.  One of them turns 18 the day before the crime, the other turns 18 the day after the crime.  

The law will stipulates the one who is 17 at the time receives a significantly lower sentence than the one who committed the crime on the first day that he was 18 years of age.  Why do we treat these two differently?  Should we treat them differently?  If not, then is it possible to even have a "bright line" rule in these cases? Can we set an age of criminal responsibility that will effectively provide for the right punishment?  What do you think?