This was my mother's father, Tevfik (later Tevfik Çelebican) possibly during the First World War when he served in the Ottoman Army. After the war, he got married, had 4 children, 3 of whom grew to be adults, operated a grocery store in a small town and drove his own car. The latter must have been a rare sight in rural Turkey during the period between the 2 world wars. Although Turkey did not enter the Second World War, Tevfik was drafted again. Unfortunately, he got sick and died at a military hospital in 1942 at the young age of 46. Therefore, in 1918, when the First World War ended, he would have been about 22. He indeed appears to have been in his early twenties in the picture.
Now, let's take a closer look at his photograph. That Tevfik was carrying a sword possibly means he was a low-ranking officer, despite his young age. His belt buckle displays the Ottoman-Turkish upturned crescent and the star above it. The object hanging next to his scabbard attracted my attention (red arrow), but I had no idea what it was until my friend Richard Greene, a military uniform buff, identified it as a sword knot.
Moving further down, we come to the footgear. Again thanks to Richard's expertise, Tevfik's leg coverings were identified as leather puttees.
I don't know what to make of the stains on his pants. My sister, who has the photograph, claims that it is the picture itself that is stained. Let's hope Tevfik wasn't posing in bloody pants after a victorious battle.
Incidentally, the sword visible in the picture, or something similar to that, is still in the family's possession.
Revision added several hours later: Soon after I posted these pictures, my cousin Mete in Turkey, a grandson of Tevfik, e-mailed me a scan of a better preserved copy of the same picture (although this one has a deep crease across it). It turns out that Tevfik's pants were not bloody after all! This copy also has the date of 337 (in the Islamic Hicri/Hijri calendar), written on the bottom and which corresponds to 1918/1919 (it is not possible to determine the exact year without knowing the month). So my estimation of Tevfik's age in the picture was correct.