In the IT consulting world, this is very common. Ethical? maybe, or not. Some firms tell their employees and contractors to use their discounts, even outside of work trips. They have negotiated with the hotel chain and if the hotel chain doesn't want others to use the discount, they generally only work through one travel agency OR require employee ID.
"so I said "I don't like that." Meaning I don't like falsely using a discount code because its discount is larger than our firm's discount code. Everybody heard."
I wouldn't have suggested bringing this up on your first engagement directly to the manager questioning his authority in front of other employees, but asking HR or others later whether something like this was sanctioned. In the end it sounds like the consulting firm had more of an issue with how you handled it than this manager.
I don't see this as age discrimination, but more of the fact that you questioned authority in the wrong place, wrong time method and got burned.

