As you age your metabolism can very easily go haywire.
Gender, age and body size and type
There are many factors that affect a person's metabolism. A short list would include: age, gender, diet, activity level.
Intelligence
Genes do play a role and have an effect on metabolism. For example if a persons family has many incidences of obesity, it is highly likely that person will also be obese because their genes will cause their metabolism to be slow.
No it doesn't
A person's metabolism is expected to drop 3-5% for each decade lived.
Yes as you age your metabolism begins to slow down and you gain weight more easily. Your metabolism is affected by your age, body size, gender, and composition. As you age your body loses muscle and gains fat which slows down your body's ability to burn calories.
Food converts into Glucose.
Beginning in middle age, the answer is (generally) yes.
Your metabolism slows down
Liver
No. Children have a higher metabolic rate than adults. Typically metabolism declines as you age.