Actually instead of asking people the informations of the requirements and stuffs, why don't you visit their websites? Private schools like MDIS, UNISIM, PSB academy are not really hard to search using search engine tools.
Age wise, like what others said it really depends on what courses and path you're interested because every courses has their very own requirement.
Like what you had asked, do SIM offers diploma courses? Thats for you to really research on rather then waiting to be spoon feed.
My advise to you is that research on it yourself, because being a student in a certain school doesn't mean that he or she would know the whole range of diploma courses, whats more the requirements for it right? Usually, they would only know about their own courses which they are currently doing.
As for your information about UNI SIM, its actually wrong, please do check the website.
For example, diploma in management studies, who said it requires a minimum age of 21?
Taken from the website itself:
Diploma in Management Studies - Part-time
awarded by SIM University Download Forms
This programme provides vital management skills and knowledge including accounting, finance, marketing, human resource management, operations management and information systems.
It emphasises practical problem-solving and decision-making skills essential to today's knowledge-based economy.
This part-time programme is conducted over 2 1/2 years. This programme covers the following 15 subjects:
English for Business
Information Systems for Business
Managing Human Resources
Managing People and Organisations
Operations & Total Quality Management
Principles of Marketing
Some components of this programme are web-based, so access to a computer is necessary.
Upon successful completion of the Diploma in Management Studies programme, you may use the initials, Dip MS after your name.
At least five GCE 'O' credit passes including English (as a first language) or an equivalent recognised qualification
At least 18 years old with 2 years of work experience http://www1.sim.edu.sg/sim/pub/pgm/s...uID=174&ID=419
Its not really that hard to search around for it.