right age for preschool education

What is the right age for children to go to play school? Is my child ready to go to the playschool? will she be able to overcome the separation anxiety? how long does it normally take for a child to adapt to school environment? These were few of the questions that trouble every parent's mind when they begin looking for play school.
        In India and Singapore ,children above 18 months of age are considered old enough to go to preschools. But is a child of 18 months age really old enough to go to playschool. In my daughter's playgroup there are few children who are not even two year old.These children stand apart from the rest of the class. Some of them do not pay attention at all to what goes on  in the class. they keep themselves busy in something or the other. At times they encourage other children to join in the fun, making it immensly difficult for teachers to manage the class. while there are few others who are unable to follow instructions. above all most of them are still not toilet trained.
            All the playschools nowadays are well equipped with good teachers, assistant teachers, variety of toys and other accessories. Even the student teacher ratio is also well managed. In singapore it is 15:1 i.e. one teacher for every 15 children plus one assistant teacher but still these younger children (below two years of age) demand more attention. Younger children usually cry for many days before finally accepting the change. For working parents it becomes a necessity to send children to playschool as early as possible but still it is good to wait and watch your child for some time and then accordingly decide. Children are very sensitive at this young age and any harm at this age can go a long way. So, considering the individual needs and requirments of the child and parent both the decision should be carefully taken. It is very difficult to point out the right age of sending children to school but parents can consider the child is ready to go to play school when the child:-
-  becomes atleast potty trained
-  manges to pick up words more clearly
-  follows instructions sometimes atleast
-  watches tv continously for atlest 15-20 minutes
-  manages to convey what he/she wants
-  tries to learn new things
-  enjoys company of other children