When is the schema validation ACTUALLY happening?

const schema = z.object({
    age: z.coerce.number().min(18)
        .refine((val) => {
            console.log('REFINED VALUE: ', typeof val, val)
            return true
        })
})

type InputType = z.input<typeof schema>

const TanstackFormExample = () => {
    const defaultValues: InputType = {
        age: 12
    }

    const form = useForm({
        defaultValues,
        validators: {
            onSubmit: schema
        },
        onSubmit: ({ value }) => {
            console.log('ON SUBMIT: ', typeof value.age, value.age)
        }
    })

    return (
              ...
            <form.Field name='age'>
                {(field) => (
                    <TextInput
                        placeholder='Age'
                        value={field.state.value}
                        onChange={(e) => field.handleChange(e.target.value as unknown as number)}
                        onBlur={field.handleBlur}
                        error={field.state.meta.errors[0]?.message}
                    />
                )}
            </form.Field>
              ...
    )
}