Googling around, this turned up pretending to be written to Dr Watson with his response:

"We're sorry to hear you've been seriously wounded at war. Can you tell us about your experiences in the Afghan wars in the 1880s?"

"The Battle of Maiwand, where I was injured, was one of our few defeats during the Afghan Wars. The British government has been very concerned about the Russian advance through Europe and Asia and we are determined not to allow the Russians to reach India, which is a part of the British Empire. At Maiwand, 2,400 of us were attacked by ten times as many Afghans. But General Roberts came to our rescue by marching his men across the mountains to rescue us."

The Lion's Mane, one of the last tales, is set in 1907 which fits with the 1880 date.

So if Watson's Afghan experiences were during the 1880s, that suggests he was born after 1850, perhaps he was not so badly trained after all.