I have one character who is an old man. Big beard, walks with a cane, occasionally forgets where hr put his teeth. My mind's eye has a perfect view of him, but my writer's eye is troubled on how to show that in the things he says. How does one make his words "sound" like those of an old man?
If there is an accent or ways of speaking he uses, there is your start. The audience probably expects him to get lost in conversation and make connections nobody else would make and get sidetracked. He probably mumbles some; mostly to himself but to others when they are near. If there is a retirement home (old folks home) near, stop in and ask if you can talk to some of the residents to help you get an idea of how the elderly conversate. Talk to as many as you can and pay attention to how they say what they say and how they get to what they are saying.
Do you spend much time around old men? In this case I'd just think about my grandfather's mannerisms and pick and choose from them. Huffs and sighs a lot, meanders around every point he tries to make, tends to go off on tangents / stories, loves comparing things now to Back In My Day (as cliche as it is).
How you write this depends on your POV. There is only so much that you can convey through your old man character's actual speech. The rest is what is observed by whoever your POV character is: for example-- I moved closer to old man Smith as he started to speak, struggling to hear his failing voice. "Tell them to get the hell off of my land," he said, his right hand trembling as he clutched my arm. "I will," I said. He pulled me in closer, tightening his grip. "Promise me," he said. My eyes met his, and I noticed for the first time how steely gray they were. "A promise!" he demanded, and a splatter of spittle wet my cheek. "I promise," I said, wishing he'd not misplaced his false teeth this morning. A silly example, but my point being that our description of our characters comes thru the eyes of our POV character. I personally don't think it works to convey an accent or "sounding old" in dialogue. Use description and dialogue tags. In other words we make him "sound" old by painting a word picture of him as old so that the reader has the same "mind's eye" as you. If you can picture it, you can write it!
