Is there a mandatory retirement age in a general sense, if yes than what would it be in your opinion?

No, it just requirement of society for creating job opportunity for the new enterent.

But, when a person think he can not further perform his duties. He is retired

There is no retirement age in my openion.

This decision should be left for the employee to decide if he is capable to do his job tasks or not.

The employer should evalute the performance of his employees annually and decide who should continue progress or terminate contract on KPI strategy and regardless to age factor, it is a question of good performance.

The "official" retirement age in UK was65, but now times have changed and that no longer applies, in fact I believe it is illegal to "force" someone to retire on the basis of age. A lot depends on a persons "ability" to continue working ie being physically and/or mentally able. Furthermore when advertising job vacancies companies are not allowed by law to discriminate against anyone on the grounds of Religion.Gender, Colour, Creed, Disabilities or AGE, candidates must be judged purely on their qualifications to do the job. There are exemptions in exceptional circumstances, but that is the general rule.

Retirement is mandatory as you need to spend more time with family.60 will be a suitable age.

As per numerous state regulations, there is60 to65 years which is now a mandatory retirement age in general sense. In my opinion it will primarily depend on the kind of work you are associated with, your comfort zone and your physical wellbeing. I know people retiring at60 and then working till75 as consultants because of immense experience and good health. I have also heard the term "early retirement" which of course does not signify below line or has to specifically relate to age but then everyone has their own priorities in life.

I don't think so. Anyone should continue to do what he/she loves doing as long as he/she can meet the agreed performance measures.