How old is 20 months?
1year and 8 months
This depends on the species. A male Red Kangaroo matures at around 2 years old and females between 15 and 20 months old. Both the male and female Grey Kangaroos can breed from between 20 months and 36 months. Most wallaby species can reproduce from 18 months old. The musky rat-kangaroo, being smaller, can breed from about 12 months old.
Kangaroos can reprodue once they are sexually mature. This happens at different ages for each gender of the three kangaroos: Red Kangaroo: male: 22 months old; female: 18 months old Western Gray Kangroo: male: 20 months old; female: 17 months old* Eastern Gray Kangaroo: male: 42 months old; female: 18 months old *she actually has to wait until the breeding season after she becomes sexually mature before she is allowed to reproduce.
That really depends on when she was bred. The average gestation period of a cow (or heifer) is around 285 days. Thus, if a heifer was bred when she was 15 months old, then she would likely give birth by the time she is 24 months of age. If she was bred when she was 18 or 20 months old, then she would calve out when she's 27 to 29 months of age.
It depends on when she was first bred. If a heifer was bred when she was 14 months old, for instance, she would have her first calf when she's around 23 months old. If she were bred at 6 months old, she would be expected to have her calf when she was 18 months old. If she were not bred until she was 36 months of age, then she'd give birth at around 45 months…