I was appointed in my late 20s in the late 1990s. I've been on BOEs that had elders in their mid-20s. I thought I was too young but had been a MS for almost 10 years so they felt that helped me "qualify". Really, being a WT elder is about doing a bunch of parts and administrative duties.I was very compentent at that duty. I never really worried about people coming to me for advice because there were others that would fill that role. And sheparding is so rare that I didn't worry about that either. And I usually managed to wiggle out of JCs.
I was the youngest at the time on the BOE and myself and the other young elder would be assigned to handle issues with the kids. Never really got called on to provide advice to someone older on life issues. I probably would have asked someone else to do it anyway as some of the other comments show that getting life advice from someone young enough to be your son isn't usually received very well.
Really, what choice do they have? Most of the guys I know that were appointed around the same time I was have either been deleted or have stepped aside. I think the burnout rate on the young ones is just as high as it is on the older ones. If they could drop the burnout rate, then they wouldn't need to appoint guys in their 20s to run their congregations.