R.I got confused when I have to omit repeated words, as follows.
1. However 70% of over 49yr olds study for interest in comparison to 18% studying for career reasons in that age group.
-In above sentence, "of over 49yr olds" is omitted. Do native speakers understand that kind of omissions? Is the above sentence correct?
Yes, it's relatively unambiguous (depending of course on the sentence previous to that one). It's also logically consistent since it compares 70% interest, 18% career, and there's only one age group mentioned. It would be a strange sentence if it ended " ... in the under 49yr group", because that wouldn't be a comparison at all.
2. The heart attack rate in women aged 65 and higher is found to be three times greater than in women aged 40-65.
-Also, above sentence omitted many things before "in women aged 40-65."
Unambiguous. What do you think it could mean?
Is the above sentence correct?
Not really. It probably means, "three times as great". Otherwise it's correct.