For our kids, coaches start them on harness at single axel level usually. A lot time for the younger girls, it isn't about fear, its used as a "reward" if they land some good jumps off-harness or something. The younger girls simply think its "fun" or they feel "important" when they get to use the harness.
My coach trains me the same way as the kids, so I wasn't on harness until single axel too. But she rarely uses the harness with me: partly I think because its a lot of 'effing effort to lift me compared to young children (I'm not fat, just tall), and partly because I don't think it helps me that much. It just becomes a "crutch" for me in most cases. I can and will jump eventually if you stop "babying" me. Currently, I'm working on getting my axel consistently, fully rotated and working on my first doubles: 2sal, 2toe, 2loop. Everything is currently 1/2 under-rotated. YAY!
However, my coach has used the harness on another adult skater who is only 2 years older than me when she was teaching her waltz jump. I'm 27 and she's 29, but we couldn't be more different. I'm a daredevil and she's very, very fearful and cautious. Neither of us is overweight or out of shape, but she's much much more fearful than I've ever been. Putting her on the harness for the waltz jump just once really did help her overcome that fear of jumping. So if you have someone who is incredibly fearful of a particular jump,
any jump, then I think using the harness
once can help get them over that fear. After that, don't use the harness again since you don't want to "baby" them or let the harness become a "crutch". Once they've done it successfully on the harness, then you know they can do it off-harness. They've just got to get over themselves psychologically. Letting the harness become a crutch won't help them with that.
P..S. I wear hip and butt pads once I started working on axel and above since you have to "commit" to falling on those jumps to see good progress. The pads are great. They give me confidence to push as hard as I can and they've kept me safe and bruise-free many times. So far, the pads haven't become a crutch for me since I sometimes forget to put them on before I start jumping. I have to make sure I remember to put them on since I know a bad bruise or hard fall will make me scared of a jump for awhile and I don't want that kind of setback while I'm still learning.