Next to Dobby, the most talked-about house elf in Harry Potter is Kreacher. Kreacher and his family are said to have served the Black Family for centuries.
From the description and mannerisms of Kreacher, we get the impression that he is quite old, yet we are never given a definitive answer as to his actual age. The closest we can come boils down to two very different answers.
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Did Kreacher Live to be 666?

According to the author’s words, “He died. Age 666.” There are a couple of problems with accepting this answer as actual, however.
Earlier that same day, Ms. Rowling had an argument with another Twitter user who accused her of being out of line for having Nicolas Flamel die at the age of 666. The reader felt the “mark of the devil” had no place in a children’s book.
The author snapped back that the reader should have read the books before criticizing her because Flammel did not die in them. It was at this point that another reader asked about what happened to Kreacher and got the short reply from Rowling.
Was she just being snippy because of the earlier spat or was she being serious? We can’t determine that and she hasn’t clarified.
There is another way of determining the approximate age of Kreacher, that would put Kreacher between 158 and 178 years old.
The Black Family and Kreacher
According to the Wizard’s Ordinary Magic & Basic Aptitude Test (WOMBAT) the average life expectancy of a house-elf is 200 years old. This would coincide with serving a family for centuries.
While we don’t know at what age the servitude begins, we can assume it is at the start of adulthood, or about the age of 20.
J.K. Rowling donated a copy of the Black Family tree for auction and it lists eight generations. The first known person on the tree was Licorus Black, who was born in 1808 and died in 1872.
As the leader of the Black Family, he would have had to be approximately 20 when he took charge. This would have been about 1828.
Since Kreacher’s family, especially his mother, was said to have served in the earliest years, we will consider that she was young at the time, maybe having been born at about the same time as Licorus, 1808.
We aren’t told how often or at what age house-elves give birth, but let’s say Kreacher was born about the same time his mother started serving the Black family. This would put him at being born around the year 1828.
Calculating Kreacher’s Age
If the above dates hold true, Kreacher would have been 151 when he first appeared in 1979. This is our earliest glimpse of him.
When Harry inherits Sirius’ estate and Kreacher, in 1996, we will have seen Kreacher age another seventeen years. This would put him at the ripe old age of 168 when he started to serve Harry.
There is mention of only one house-elf as appearing older than Kreacher. Hokey, who served Hepzibah Smith, holds that position. When you take into account all our calculations about when Kreacher may have been born, and the average life expectancy of a house-elf being 200 years old, this all makes sense.
Conclusion
While we can’t know the exact age of Kreacher as he appeared in the series, we can logically assume that Rowling was being sarcastic during her tweet and that Kreacher was approximately 168 years old when he began to serve Harry.
Leaving a margin of ten years, either way, it is a very safe assumption that Kreacher was somewhere between 158 and 178. This would fit with what we previously know to be true about the average life expectancy of a house-elf. Sources