[jddj]

Why? Plenty of elementary aged kids use the tube

[djtango]

Do they? I don't think I've seen anyone younger than 12 unattended on the tube

[Symbiote]

I used to live round the corner from a private school in London (ages 5-16 or 5-18, not sure).

They were never on the tube with me, but plenty of children would be arriving at about 8:15, as I was departing to get to work for 9:00. Private school children are more likely to have a longer journey to school, therefore less likely to have a direct bus, and presumably more likely to have parents who will pay for the tube/train. (Only buses are free for children.)

[jddj]

The DLR at some hours between some stops is practically a school bus.

I guess it's possible they're small but not <12yo

[prmoustache]

Are you calling 12y olds small? That is an age many female already have their periods. My youngest daughter is 12y old and almost 1m70 (5'7").

In some countries and even US states this is already an age you can marry.

[AnnikaL]

Without court approval, no one under 16 can get married in any US state.