Our bodies change as we get older. Also, we may have not built up an immunity to them. Possibly because we did not receive allergy shots as children against the specific thing.
This varies from person to person. Some people may have on their list to sky-dive, others may want to take a trip to Antarctica or to China. There is no definitive answer for this question.
First thing that one would require to become a US citizen is the Green Card. For that they need a sponsor. Once obtained a Green card, then they have to be a green card holder for 5 years ( 3 yrs for...