I personally started at around 4 or 5, not sure. An uncle babysat me and had me suck him and swallow his cum. I remember thinking of it like a secretive chore I didn't want to do. But then one night he brought his friends over and all I remember is a lot of faceless cocks, excitement, and happiness at helping so many men cum.
To answer the OP's question, I don't know when the best age is to start. I might not have hated sucking cock at first if I had started earlier. Obviously, I grew up to become a cocksucker so earlier was better for me. But even though I don't think I was abused, there's a reason why it's illegal to do what my uncle did. Some 5 year old boys, later cocksuckers or not, might have been traumatized and hurt after sucking off a roomful of strange dick.