nazri,
SIA will call you up for the interview even if you have combined science and you meet all the requirements as stipulated by benedictz. Your eye sight doesnt seem to be a problem, go get it tested and make sure you dont exceed 6/60.
The age limit of 26 is not decided by SIA they would rather take you as young as 18. However, RSAF has given them some leeway to take cadets in starting from late 24s and 25. I guess you can submit your application if you're one or two month away from your 25th birthday. No harm.
For those who have gone for RSAF pilot scheme and got dropped, you are eligible to join SIA even before 24. But the way the candidates are shortlisted in SIA, they wouldn't know if you have gone for the RSAF scheme or not. Best if you write in and clearly state that you have gone for RSAF and not made it.