Actually what they want to see is an all-rounded individual. The first they look at is your academic qualifications, the degree and where you got your degree from. The chances of non-degree holders are very slim. Secondly, they would look at the consistency of your results, whether you have improved over the years or just kept constant. Thirdly, your ECA and activities that you participate in. They also look at the subject you took. If you are in the science field (medicine, sciences, engineering), you stand a pretty good chance. It shows that you can adapt to the scientific material and logic, which there are lots of when flying. O and A levels will only be judged upon to make sure you can handle the basics of pure science (shows how you can cope with basic science). A fellow pilot of mine retook his O levels, did not do very well, got into poly then into NTU engineering but still managed to clear and now he's happily flying.
Flying experience is a plus but not a main factor. If you have flying experience, good for you! It keeps you ahead of many others with no experience at all. It shows you do have the interest and willing to spend that amount of money on this interest. Those with PPL, you do have to produce your medical and such too. I went in without flying experience and got in because many others too don't have that. It's hard to get that in Singapore unless you have the time and money.
There are people who get in at 30 years old. I know a few who got in when they are 31-32 years old. One of them was a doctor. He finished his housemanship and realised that he hated working in that industry. Managed to get in at 31 years old.
For those who is going for the 1st interview...
Carry yourself well. That's the most important. I've heard many not being called back because they did not know basic manners. If you did not clear the first interview, don't give up hope. If you do get through the first interview, you already have one foot in. It's just your attitude and knowledge already.
Those applying, don't give up hope. Don't be nervous. Just go in there and have fun with the interviewers. (as in treat it like sharing of knowledge with them about yourself and flying and not treat it as if you are being interrogated). Be light-hearted, bring a good smile, and good luck!