My DD7 sits on my lap if an adult is standing, as that is how I was brought up, and I would be uncomfortable any other way. ...I think generally children under 7 should be on laps if adults want to sit down
If DD7 was twins, or had a younger sibling, what would you do? You only have one lap. It's a safety thing, there have been times when I have been glad of my husband shielding me and the baby from those who press onto the train as if we lived in Tokyo, a small child would be in serious danger standing. Even in a less crowded train, they don't have the experience/skillset to balance themselves, and can't always reach any sort of handhold. my son reached the age of wanting to stand, and held on to a vertical pole, and ended up snashing his face into it at a stop.
In my mind, anyone unable to stand securely should sit. If a lap is a available, children have that option, but it isn't required. Otherwise, older folks, younger children, some people with disabilities, and some pregnant women. Except for the children, it isnt always obvious if the passenger cant manage standing. I try to catch their eye/ask, cause nothing is more annoying than getting up with the intention of letting them have your seat, and they dont want it and someone else takes it before you can sit back down! Ok, it is more annoying if they did want it and someone takes it.