In Ontario, at what age and grade do children graduate from high school? I have read answers giving both 17 and 18.

Well, it depends. There's a range of birth dates in any given intake of kids.

For example, even if we had not moved to Australia and thrown their ages further out of kilter with respect to their classmates, our two sons would have graduated from high school at different ages. Our older son's birthday fell in such a way that he was practically the oldest kid in his class, and he would have graduated at 18. Our younger son's birthday fell in such a way that he was practically the youngest kid in his class, and he would have graduated at 17.

That was in Calgary, but the same would have held true in many jurisdictions in Ontario.
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In Ontario children go to school for 12 years and graduate with a High School Diploma after that time so if a child enters while 5 years of age then graduation would be at 17 and so on.

I thought it was 19??

Correction. I have been told its now 18, it used to be 19. Also you can't leave school until the June of the year you will be 18 in.

They changed it from 19 to 18 and that year there was a "double cohort" ie twice as many graduating.

Yes, Ontario used to have a grade 13 which, as you indicated, was eliminated.

The double cohort year at the high school level was 2003. This had a cascade effect at the university level, when many of the 2003 high school cohort graduated with bachelors degrees in 2007.
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It could be both. Generally school ends in June so those born before June are generally 18 and those born after June are generally 17 when they graduate.