Lots of things CAN cause feet
to swell. For example: hormone changes, heart disease, kidney disease, fluid overload, endocrine disorders, diabetes, burns (even sunburn), inmobility, and electrolyte imbalances are a few things.
It's finding out what causes YOUR feet to swell that's most important for each person. A 70 year old sitting at a computer is likely to have valve deficiencies in his leg veins and be fairly inactive (but is by no means necessarily true!).