You are looking for a very simple answer to a very complex issue. There are 3 factors in remaining healthy. One is lifestyle, such as weight and smoking, another is age, and the third is plain luck (including genetics.) A person can be young and 600 pounds and be quite healthy. Another person can be thin (or fat) and suffer a slew of age related problems, especially orthopedic mobility issues. Or a person can develop issues over the years that can be complicated by weight. So gaining now may have no consequences, but if you get older or hit bad luck there is no guarantee you will remain healthy.
4 years