I think the title of the thread is problematic: when to MAKE an unwilling kid ride. And I think that's why you've gotten a lot of "never" answers. I think PPs have offered ways of CONVINCING or PERSUADING kids to ride, but just forcing them to do something they don't want to do - something that's unnecessary and supposed to be fun - is what's rubbing people the wrong way.
My daughter is on the bubble about Everest on our next trip. I've shown her youtube videos and let her read what others have said about the ride, but the decision is ultimately up to her. I don't want her to miss out on something she might like, but the truth is - it's no big deal if she wants to skip it.
Interesting PP about escalators and elevators. My daughter hates escalators. She and my husband went on a trip to Peru. She was 11 then, I think. I knew she'd encounter escalators in the airports and it was a tour group, so freaking out and finding another way might not be feasible or comfortable. So in the days leading up to the trip, I took her to the mall and we rode escalators. I promised her a Starbucks drink after each "session". She still dislikes escalators, but she wanted to practice each day because of the Starbucks drink. Long story short, she got over her fear enough to make the trip go smoothly, though she still marveled at the other kids on the tour who would ride up and down the escalators for fun. LOL. Just not her thing.
Anyway, I think that's different. I do think conquering fears is important, but riding a roller coaster is just not a necessary part of life.