The tradition of receiving a telegram from the monarch on the 100th birthday of a British (or Commonwealth) citizen dates back to 1917 and the reign of George V. They are also sent out for Diamond wedding anniversaries.

These bicycle-delivered telegrams were written by a secretary of the Queen. Originally, there were very few sent out, as life expectancy was much lower at the beginning of the 20th century. There are now a few thousand a year.

In 1982 telegrams were stopped within Britain, and restricted abroad to special occasions. A secondary message system using the Post Office remained, and the Queen switched to using this for her congratulatory messages. To keep it special, she changed the style of the letter, as if it were written by her instead.

Compare a 1958 telegram

"The Queen is much interested to hear that you are celebrating your 100th birthday and sends you warm congratulations and good wishes."

and a 1999 message

"I am pleased to know that you are celebrating your 100th birthday. I send my congratulations and best wishes to you on such a special occasion."

Messages are sent out for a 105th birthday and then one a year, and for 65th and 70th anniversaries and then one a year.

The message itself is now a glossy printed affair in a blue envelope, with a coat of arms and scanned signature of the Queen, along with a photo.

I will never get a telegram from the monarch because

1 - The monarchy won't exist by the time I've been married 60 years or have my 100th birthday

2 - I won't live to a hundred

3 - Nobody would live with me for 60 years.

much info from

I look forward to being proved wrong about any of my 3 points above; When I'm 100 I'll check everything9 and realise I was wrong.