Just by way of follow-up ...
My (early teen) kids flew by themselves YYZ - FLL earlier this week and I enlisted the help of the YYZ concierge office to shepherd them thru check-in, US Customs & Immigration, Security etc.
I arranged it in advance so they were prepared. As usual they were great - obliging, considerate and efficient. Just love the service!
Lornag - further to my earlier post I have met the concierges at select US destinations - LGA and LAX. They are pretty visible if you know what to look for (dressed in black uniforms with 'concierge' tags on lapel).