Nope. It never is too late.
In the first place, you said that one reason why you didn't take up
the RSAF offer is because you would have to go for BMT too soon.
Well, I suggest that you take some time off to really think long
and hard about it. You don't have to take up their offer now;
surely you can always enlist again a few months down the road.
Unless if I'm badly mistaken, of course.
Another course of action for you to take would be to go to poly and
enrol in aeronautical engineering. I advise you to consider this
route only if you're really, really determined to work in the line
of aviation. Because you realise that if you follow this route, you
would have practically wasted two years in JC doing something
totally unrelated to aviation.
But if the dream is big enough, the facts don't matter. It would
take around six years from poly to uni to get a degree in
aerospace. If you're really committed to making your dream reality,
then it can be done.
Cheer up, yah. It ain't over till it's over. 
