This dress is beautiful! It's only the second MyAG outfit I've purchased in the past 2 years, the currently available outfits just don't appeal to me much. I really like everything about this one though and do not at all think that it looks too young. I would have worn a dress like this at 13 or so.
The dress seems high quality to me. Fabric does not feel cheap and the sequins are sewed on well. The underskirt/slip thing is nice and a great addition, it would definitely be way too see through without. It helps the skirt lay nicely as well. I'm not a huge fan of the sash going around the middle- the serged seams are kinda ugly and the sash lays oddly on my doll, but it's not a big problem.
The shoes are pretty, fit well, cutely decorated. I do agree they look a bit like slippers though! Still look nice with the dress.
The barrette seems high quality. It's metal and stays in the doll's hair fine. It would have been nice if it could hold a bit more hair though. The back of the barrette is pretty long, but only about half that length can actually hold hair and the rest sticks out beyond the barrette which is a bit unsightly. A little odd, but still a cute addition and it looks nice with the rest of the outfit.
Overall, I would definitely buy this outfit again!