Hi mama, I haven't come across a specific support group just for first-time mums over 40 in Singapore, but honestly the regular mum groups at polyclinics and community centres are mixed age anyway and you'd find plenty of older first-timers there too. Have you tried checking with KKH or your neighbourhood CC, or even Facebook groups for your estate? Sometimes the best support comes from just connecting with other mums going through the same thing, regardless of age bracket.
I hope you get to find the support needed here!