How old was King when he died?

It seems like you might be referring to a different historical figure, as Martin Luther King Jr. was indeed 39 years old when he was assassinated.

If you're mentioning a king who died at 51 in his home in London, that sounds like it could refer to King George VI. He died on February 6, 1952, at the age of 56 in Sandringham House, which is not in London but in Norfolk.