Are old people allowed to weight more than young people?

Yes, old people can have about 10-15% more body-fat without problems, it's because as you age you need more reserves to stay healthy, skinny older people usually have double the health-issues compared to the heavier older people.

The stats also suggest that overweight BMI is healthier too for everybody else, people with a BMI of 25-30 live longer, get injured/ill less, recover faster than other weight-groups, and live more active lives.