'Fancy some sex?' he says, quite loudly. I glance up to see who he is talking to and realise with a sinking heart that it's me.
'Pardon?' I must have misheard. I meet his eye. He is looking down at me - he can't be more than 13. Dark skin, thin, dirty. Dreadlocks which hang, fuzzy and matted, to his little boy's waist, where a silver skull snarls dangerously from his big boy's belt.
'Sex. D'you fancy some?'. His face levels with mine as he tries to force half of his bum on to my seat.
'Excuse me'. I shove him off. What's he on? I look around, embarrassed, but no one reacts. Either he looks my type or everyone on the entire platform is deaf.
Did I say embarrassed? Because actually I'm not - it is more a trickle of dismay, because he is such a kid. Because this proposition is not a proposition and clearly has nothing whatever to do with sex. It is merely another of those meaningless dialogues, all too familiar and vaguely threatening, enacted by a nutter. Only this one isn't threatening, because this nutter is a child.
And look at me. I'm old enough to be his mother - in fact I am a mother of three kids not a lot younger than him. I have a Peter Jones carrier bag (visibly) full of Pampers at my feet, my make-up has evaporated and I'm starting a cold. Did he say sex?
'No thanks,' I tell him and look away.
He gives up trying to sit with me, thank God, and stands staring at the pigeons. He looks wasted, probably a glue-sniffer. His face falls to nothing beneath his high, pretty cheekbones and he dashes his sleeve now and then across his constantly running nose.
A train arrives, unfortunately not mine. I consider getting on anyway, but it is broad daylight and I'm damned if I'll go out of my way for a crazy.
'I've got a place around the corner,' he croaks. 'We could get some cans, have a smoke. You like to smoke?'
I laugh, exasperated. 'How old are you?'
'Fourteen.' (So we're talking 12, 13 max.)
'Shouldn't you be at school?'
'It's Sa'urday.' It is. 'Whassa madder? Don'tcha like sex?'
'Oh come on.' A touch of compassion makes me suddenly brave. 'What do you think I'm going to say? You know I'd never come back with you.'
Maybe he clocks the bravado, because he gives me a long look and moves away without a backward glance, his shadow trailing after him - if not exactly Peter Pan, then certainly one of the Lost Boys.
In this city, they are everywhere, these lost kids - mostly boys - who look as if they need looking after. And I don't mean teenagers, either - I mean real children, the ones who wander up and down our street at dusk, asking to clean our car, collecting sponsorship for a school walk or swim. There are the two little boys who rush over to soap my windscreen at Vauxhall Cross one summer's afternoon - not the teenagers I've seen all summer, but a couple of kids barely out of primary school, who look as if they are badly in need of a carton of milk and an early night.
Their weary, pinched expressions break my heart. I know this look - I've seen it in the drained face of my five- year-old after a long day at school. He gets a cuddle and maybe a spoonful of Calpol; these two just stand there, shattered, as the rush-hour traffic roars by.
And of course I give money to the nine-year-old West Indian boy in a thin nylon windcheater who knocks on our door after dark in the driving rain, selling raffle tickets for his school fair.
'Do you really have to do this at night?' I ask him.
He replies that it is when most people are in. Any double-glazing salesman will confirm the truth of this; but while most charities' small print discourages children from collecting door-to-door, isn't it time schools also gave it some thought, if parents can't?
And why do we accept this - little children, promoted to teenage status and allowed to wander the streets, knocking on doors when they ought to be in bed?
Personally, I don't want an eight-year-old cleaning my car - not unless he is my own eight-year-old and I'm showing him how to do it. It doesn't feel right. Neither do I want raffle tickets from a nine-year-old whom I have to send back into the darkand the rain.
Have children always been doing this sort of thing, or are we all just turning a blind eye and allowing it to get worse? Or is it simply that you are that much more aware once you have small children of your own?
When I tell my Victoria station sex story at a dinner party one night, I find it oddly appalling that - once the laughter has stopped - I'm still regarded as the potential victim of the situation.
'Still,' they soothe, 'you were safe really, weren't you? He couldn't have done anything on a Saturday afternoon and with all those people around.'
'Christ]' I explode. 'You've missed the point. He was about 13 - wasted, small, exhausted] We're talking about someone who's barely reached puberty] I meant it wasn't safe for him, not me]'
They blink. Uneasy, baffled laughter. Are we talking an attitude problem here or what?Reuse content