Not true. Some JG17 (Assistant Director equivalent at SingHealth) are not HODs.

Second part is probably true for an average worker but I have seen JG17 at 40 yo. They are usually the high performers.

I myself was a JG15 (Manager equivalent at SingHealth) at 33 yo when I joined (did Ph.D. for 5 years and industry R&D for 3 years), so there were many staffs who were older but more junior in rank than me and reported to me.

Promotion in Public Healthcare Institutions can be difficult even if you perform well consistently, so I left after 2 years for better prospects elsewhere.