Deciding on the age of Hercule Poirot can be a little tricky. Some say he lives to 125 by the time of his last novel (found in various reference books and amongst Agatha Christie scholars/experts). Christie said in her An Autobiography (finished in 1965): "What a terrible mistake I had made in starting with Hercule Poirot so old--I ought to have abandoned him after the first three or four books, and begun again with someone much younger" (italicized "old" was included in text). She also made a mistake, she said, with making Poirot a retired police offer. She also says in her autobiography: "Not too young a one. What a mistake I made there. The result is that my fictional detective must really be well over a hundred by now." Indeed Hercule Poirot was! Let's discuss his age a little further.
In Three Act Tragedy, Poirot said he was to retire from the Belgian police force at the time WWI began. That meant if the age of sixty was for retirement, then Poirot would have been born around 1854. That meant Poirot was a refugee in England at age 62 roughly (in the year 1916) when Arthur Hastings saw him at Styles. During the period of his "retirement" (if he even did retire), the first World War was being waged. Did Poirot get sidetracked, perhaps brought into the resistance? That is the question Anne Hart poses in her book The Life and Times of Hercule Poirot. As recounted in the Poirot novel Murder on the Orient Express, Poirot owed gratitude to a French general for saving his life. The General had mentioned again to the detective "of France, of Belgium, of glory, of honor", but stops just short of mentioning World War I. Was Hercule a resistance fighter, having escaped to France? King Albert I of Belgium himself in the short story "The Kidnapped Prime Minister" recommended Poirot's services: capable of saving a missing prime minister from the enemy. Is it possible that Poirot provided services for the Belgian throne before? Maybe Poirot gained incredible skills in the resistance, not just in the police force. This is, of course, harmless hypothesis. If this were all true, then how exhausting it would have been for our Belgian sleuth!
That would make sense for his desire to retire before The Murder of Roger Ackroyd when he was 72 (in 1926, when the story was published). He vowed to retire after his "Twelve Labors" (the stories were published in 1939-1940, but collected in 1947). He was already aged 93 then! Poirot clearly went into semi-retirement, taking only cases that intrigued him. That would have him aged 121 when he appears in his last story Curtain (published 1975).
Christie was careful to age Poirot slowly (she was wise to!). She didn't know when Curtain was to be published (it had been in a vault for some 30+ years). Norma Restarick in Third Girl (1966) changes her mind about speaking to Poirot because she found him to be "too old". Indeed! In the 1969 novel Hallowe'en Party, Poirot admitted to ex-policeman Spence that they were "both old men"; however, Poirot confessed that he still dyed his hair black! Agatha Christie met two necessities in choosing Poirot to dye his hair black every now and then: 1) it continued to keep the characterization and vanity of the character and 2) it aged Poirot slower in years because he didn't display gray hair.
In the book The Agatha Christie Companion, Sanders and Lovallo say one could use an "age formula" to determine Poirot's age. They say Christie employed a 3 to 2 ratio in her books, meaning that Poirot ages about 2 years for every 3 calendar years. I calculate then that if Poirot was 62 in 1916, when he came across the English Channel, he would've been 101 (so young!) at the time of Curtain. My calculation only assumes if the counting started from 1916 (when The Mysterious Affair at Styles was written).
The authors of The Agatha Christie Companion say this same 3:2 ratio/method can be applied to Poirot's friend Captain Arthur Hastings as well. Calculating Hastings' age, they said, would make him in his 60s in Curtain. Hastings clearly states in The Mysterious Affair at Styles that he was 30 years old. That was 1916. So . . . if I started calculating at 1916 and I assigned him the age of 30, his calculated age would be 69 for the novel Curtain. I'm so at peace with this. I always pictured Hastings in his 30s when he first narrated Poirot's cases. I always felt that a difference of at least 30 years were between him and his friend Poirot, too.