Public statements by the defence minister on the fastest
possible rate of advancement state that General rank can be had by
the age of 40. Reference: MG Chan Chun Sing.
That said, Lee Hsien Loong was the youngest General in SAF
history at 32. Even more unusual, this was in 1984 when SAF
scholarships were extremely rare and "selective".
There are no 18 year olds with General rank, but there are those
on track for it. That's the scholar scheme for you. You might find
this local blog very informative
kementah.blogspot.com