What do you seriously think of this outfit for Sunday best/church? age 13?

It depends on what kind of church you go to and what age the population is mostly made up of. It looks a bit old-fashioned to me if you go to a nondenominational or other more modern church. I mean, it's conservative, which I suppose is good for church...but like I said, if you go to a church with a more modern style of worship and a younger age group, I would definitely not wear this. Unless you really like it, in which case, wear it. I hope this didn't sound mean or anything, I was just being honest. I hope I helped, at least.