DS is 14 and has never tried to call me by my first name that I can recall. He calls DH by his first name because he's DS's stepdad. He calls my ex "Dad."
My mother is Mother. When I was 11, I desperately wanted to transition from calling her "Mommy" to something less childish. I asked her what she wanted to be called. She chose "Mother." Said she never pictured herself as a "Mom." Truer words were never spoken. I have never in my life called her by her first name unless I am speaking of her; for example, referring to myself as FirstName's daughter. She refers to herself as "Mother" to me, "Grammy" to DS, and FirstName to DH. My brother decided a couple of years ago (at age 43) that he didn't like calling her "Mother" and insisted we should both start calling her "Mom." He can do what he wants, but she is "Mother" to me. My dad was always "Daddy" to me.
I have three stepsons. The two oldest generally refer to DH by his first name when they are speaking about him. They do the same in conversations about their mother. No idea why. The youngest always refers to his parents as "Mom" and "Dad." I have heard the two oldest speak directly to DH by first name when they think it's funny.
My aunts and uncles were always Aunt FirstName or Uncle FirstName. I think I was in my 30s when my mother began referring to them by first name only. I still tend to use "Aunt" and "Uncle" when I speak directly to them. My grandparents were Grandma or Grandpa LastName. Even when we spoke directly to them, we still used their last names. I would never in a million years have referred to my grandparents by their first names.