There are a number of reasons for this. Below are several common possible causes:
1. If you are under 21 and have a history of Diabetes in your family, you could be showing the first signs of Type 1 Diabetes (often called 'Infantile Diabetes'). However, within the last few years, the incidence of younger people developing Type 2 (often referred to as 'Maturity Onset Diabetes') has been on the rise due to the overload of sugar and refined, prepared foods in our diets.
2. If you have no history of diabetes in your family, you could be drinking too much caffeine. Not only coffee and tea have high levels of caffeine, but lots of 'energy drinks' are loaded with it, as are many soft drinks (sometimes called soda drinks). These beverages put a heavy burden on the kidneys (the caffeine stimulates the kidneys to work overtime, as does excessive alcoholic drinks, and the result is very frequent urinating.
3. If you are taking medications, or are on stimulants of any kind, these can also have a similar effect.
4. Pregnancy can cause it, as well as simply drinking excessive liquids (excessive thirst can be a sign of the onset of diabetes, so check with your doctor).
5. If you have ruled out all of the above, or are still not sure, you should make an appointment with your doctor to make certain that a more serious condition is not causing the frequent urination.
The above list is by no means complete. If you have concerns, you should consult your doctor.