This is what I am thinking, please correct if something is wrong, I am learning too.
1) by the time they
had reached adulthood, 98 percent of men and 75 percent of women born in the country
have been attacked by the birds
mismatched verb tense2) by the time they
reach adulthood, 98 percent of men and 75 percent of women,
who were born in the country,
had been attacked by the birds
incorrect verb tense and modifier who modifies the noun that immediately precedes it, in this case women3) by the time they had reached adulthood, 98 percent of men and 75 percent of women born in the country have been attacked by the birds
Corrected all the errors4) 98 percent of men and 75 percent of women
that were born in the country were attacked by the birds by the time they reach adulthood
unnecessary introduce one more that clause5) 98 percent of men and 75 percent of women who were born in the country, by the time
they reached adulthood had been attacked by the birds
awkward construction as well as past tense verb is required as the sentence is in past perfect tense.