No, it is not too late to join at age 26.
You will be assigned as how SAF needs you. May be back to BTMC
as officer again, or may be in Gombak tomolo.
You dont really get to choose the area you are in as you are in
the "training" line.
(I dont like SAF officer/regular any way, especially BTMC
officer)
BMTC officer are considered low grade (You probably get B/C
during your OCS to be posted to BMTC. You dont get to specialised
training in other vocation, so you have to undergo the shit (ex:
GCC) again if you get to be posted to guards. Support line (ME) you
are out as you dont have an engineering diploma/degree. Promotion
is also very hard because of the lack of cert.
I suggest you put up the application and see if SAF still wants
you. Dont wait.