@shyong
Welcome to PPrune.
Like what Stallone have said, If you are adventurous, you could apply now to test the system and see if you get rejected for being underage. That way, we'll all know if SIA takes in Singaporeans below the age of 25.5 who also have been rejected by RSAF. If you do get selected, then good luck and congrats! I guess you could really forget about completing your studies at SMU.
However, I would choose to complete the degree first before making that an application to SIA. 23 to 25 isn't really a long time. Take it as 2 over plus years for you build your maturity and prepare for the interviews. Patience is virtue.