Today King Charles III is 77 and the fourth oldest reigning British monarch

Today King Charles III is 77 and the fourth oldest British monarch, after his mother Queen Elizabeth II, Queen Victoria, King George III and King George II. 

The Duke of Windsor, formerly King Edward VIII, lived to 77 and 11 months but had abdicated long before.

If you are a Jacobite, King James III and VIII lived in exile for 77 years and 5 months before being buried in the nave of St Peter's in Rome with all the honour due a King of England, Scotland and Ireland. 

King Charles III was the oldest monarch to ascend the British or English or Scottish throne, though had the Electress Sophia not been caught in a shower and lived a month later she would have been older.