Louis XIV had the title of King since the death of his father and having a regency doesn't detract from the length of his reign.
Candace had some good examples of monarchs that came to the throne in thier youth and had a regency.
But at the other end of the spectrum is George III. The end of his reign is counted with his death in 1820 and not when his eldest son, the future George IV, became Prince Regent in 1811.
I'm just a traditionalist who prefers to count the beginning of a new reign as of the date of death of the last monarch.
You and pretty much everyone else with a grasp of the definition of reigning dates!
http://europeanroyalhistory.wordpress.com/364