Hi. Just seeking clarification - what kind of tickets to the Colosseum are U.S. teenagers under 18 eligible for?
Pretty well explained here https://www.coopculture.it/en/colosseo-e-shop.cfm . The reductions apply to EU citizens and non-EU citizens under 18, I.e., they are free from what I can tell on a quick run through.
The detail not exactly explained on that page are the ticket prices for tours. While under-18's of any citizenship enjoy free entrance, tour prices apply for children 12-17. For instance, tickets for the Underground+Belvedere tour in English are € 9,00 +€ 2,00 reservation fee for children aged 12 and over. Children under 12 are free +€ 2,00 reservation fee.
So, go to the specific page for each tour to see prices.
There is a very helpful sticky at the top of the TripAdvisor Rome forum page that answers a LOT of questions about Colosseum tour bookings. The author will, I assume, make any updates needed for 2019, and is an occasional poster here on the RS forum as well.