In my opinion, letting a seating to someone else will be in this order of importance: frail elderly (who stood wobbly and holding grab-pole for support), last trimester expectant moms (who seriously have backaches and swollen feet), parents holding babies (less than a year old who obviously can't stand on their own; but without a baby pram on sight), someone holding a clutch (or walking with a limp, but could still hold steadily on a grab-pole), elders especially grandmas (with grey hair, but still sturdy, 55 to 65 years old), and grandpa (55 - 65 years old).
I will not let a seat to children (who are 5 to 12 years old) because they have the lowest center-of-gravity and unlimited power. Furthermore, they need to be beefed up and not whine for being "forced" to stand too long (especially when they did not complain about running like crazy around children playground while refusing to go home). Aunties and uncles in their early 50s who could still run into the MRT to grab seats and stood up early (while the train was still moving) so that they could inch nearer to the exit door for speedy exit. I have seen rare incidence (not common but it happened) where aunties/uncles competed (like athletes) with expectant moms (slow-mo and bulky) for seats without guilt. Probably such incidents molded my perspective towards the need for prioritizing my offer for seat to the needy in MRT.