Excess accumulation of fat tissues beyond the limitation of the skeletal and muscular requirement, is termed as obesity. As a child grows to adolescence & adulthood, there must be a steady increase in the weight in proportion not only with the height tables which are found to be based on average weight of normal persons of around 24-25 years of age and in relation to their height. Because that is the prime age when the skeleto-muscular structure is well developed & physical adjustment to adulthood is at its peak. The vital organs would have reached optimum capacity to carry out their functions. So any increase in weight as one age must be in keeping with the average of what it was at the age of 25. If not, it is always detrimental to good health. It is also important that persons of slender built should not weigh as much as persons of sturdy built having a broad frame & heavy muscle, even if they are of the same age & height.
Se people do not put on weight but merely become obese for fat. At first their weight seem to be absolutely normal & constant for nearly 15 to 20 years too. But, if there is a significant change in their activities where their muscles are exercised to a lesser degree, the muscles do degenerate or get displaced by fat. After some time, the muscle degeneration slows down. But fat does not displace the muscles to keep the weight constant. Instead it gets deposited outside the muscles under the skin making one obese as well as overweight.

It is not desirable for a person to gain weight, as he grows older. During old age the bone mass in the body begins to decrease and there is a constant thinning of the intervertebral discs which results in reduced height. Muscles fiber also reduce and as such an increase in weight is not desirable.