I do not find the request to be unusual or out of line either. Many barns make arrangements to use boarder’s horses in lessons, but usually there’s a quid pro quo/some advantage to the horse owner, not always a direct credit to board. (To the poster who said their barn gave a $25. credit for use of a horse in a lesson - where are you located and what’s the lesson fee? That’s a very, very, high figure.) I frequently had boarders ask to have their horses used in lessons so they could stay in regular work or as a cheap substitute for training board; but the horses were not always suitable for the lesson program.
I also think it’s perfectly reasonable for the OP to decline to have the horse used. Her horse, her call. And from the description of the horse, probably the right one.
The posters who pointed out that the barn pricing structure may depend on each stall generating some additional income in lessons or other services are right on - many, many show/lesson barns are structured this way. However, I would think that they’d look at the OP as a package deal - more income generated by the younger horse, less by the older horse.
So unless the barn and/or trainer has given you other reasons to distrust them, their horsemanship or their respect for your boundaries, I don’t think there’s anything untoward about this situation. Just say “No, thank you.” and move on. I guess there’s a slim chance that at some point they’ll want to boot the older horse out of the stall in favor of better income producing boarder, but I would cross that bridge later.