The question of how long people do or should sleep for is one that has a number of different answers and many passionate proponents. In reality, there are a number of different answers, as we are all different and even the same person will have different sleep requirements throughout their life. In the end it all comes down to what is right for you at the time.
This is not to say that there is not an average length of time, just that it is different for different ages and different groups of people. For a normal healthy adult, the average length of sleep is around 7-7.5 hours, for a senior citizen it is about 6.5-7 hours, while for a young child it could be anywhere between 8-16 hours depending on the exact age and for a teenager it is more like 7.7-8.5- though anyone who has a teenager will know that they would like to get far more than that!
At different stages in life we need different amounts of sleep, both for physical and mental reasons. The most important thing is to have regular sleep patterns and habits that leave you feeling rested and recharged each day. Everyone has different needs, so follow what works for you.