I always pictured Gurney being older than Duncan, and Thufir being older than Hawat. But given they all have supreme fighting talents, they must have achived some minimum age to have learnt these, yet still be young enough to be active. I think in the original book life expectancy was still short, even with melange, say 100. So I figured a range of 30's to young 60's for the 3 of them.
As for Duncan's appearance, I agree Moama is probably not the right guy. Perhaps the right heritage, but he's too big and bulky, I always pictured Duncan to be strong but slender, as I imagine knife and sword fighting is more about agility then strength.
As for how he's described in the books, I found the following during a quick search through my pdfs. Not fully comprehensive though, I probably missed a few.
Paul studied Idaho, marking the feline movements, the swiftness of reflex that made him such a difficult weapons teacher to emulate. Idaho's dark round face turned toward Paul, the cave-sitter eyes giving no hint of recognition, but Paul recognized the mask of serenity over excitement.
Nearby, more an outflanker than a member of the group, stood Duncan Idaho in glittering dress uniform, flat face unreadable, the curling black hair neatly combed.
Idaho's dark, round face was drawn into a frown. His hair, curling like the fur of a black goat, was plastered with dirt. A jagged rent in his tunic exposed an expanse of the dress shirt he had worn at the dinner party earlier.
His attention remained on an orange-robed attendant whose blindly staring metal eyes looked neither to right nor to left. This attendant walked at the right front corner of the Ambassador's troupe like a military outrider. A rather flat face beneath curly black hair, such of his figure as could be seen beneath the orange robe, every gesture shouted a familiar identity.
Turning toward the ghola, Paul said: "Hayt, is that your only name?" A serene smile spread over the ghola's dark features. The metal eyes lifted, centered on Paul, but maintained their mechanical stare. "That is how I am called, my Lord: Hayt."
The outer appearance remained similar -- the black goat hair over sharp dark features -- but in the long years since his awakening from the ghola state he had undergone an inner metamorphosis.
"I'm told you wish to help me," Jessica said. "As odd as that may seem," he said. "But didn't Alia send you to abduct me?" she asked. A slight raising of his black eyebrows was the only mark of surprise. The many-faceted Tleilaxu eyes continued to stare at her with glittering intensity. "Those were her orders," he said.
Idaho opened the ornithopter's door for her, helped her into the right-hand seat. As she moved past him, she saw perspiration on his forehead where a lock of the black goat-hair lay tumbled. Unbidden, Jessica found herself recalling that head spouting blood in a noisy cavern. The steely marbles of the Tleilaxu eyes brought her out of that recollection. Nothing was as it seemed anymore. She busied herself fastening her seatbelt.
His hair was still the black cap of karakul, the features fixed in stony sharpness with high cheekbones.
The child on the lawn appeared to be about twelve standard years of age, but appearance could be deceptive with a ghola not yet awakened to his original memories. The child took that moment to look up at the watchers above him. He was a sturdy figure with a direct gaze that focused intently from beneath a black cap of karakul hair.
The blocky gholachild from Gammu's long-dead Keep had become a lanky man: dark skin and hair. The hair was longer than when he had entered this no-ship on the last day of Dune.