I'd say that Julia and Maria were out at 18 - I think it would have been odd if they weren't.. but it seems to me that "things change" for F. because of hte Crawfords arrival. F does not go anywhere except iwth Lady B, and Lady B doesnt' go anywhere, excpet problaby to the parsonage so F may go with her... but the arrival fo lively young people at the Parsonage, who visit the house and get to know Fanny, seems to push the Bertrams towards "bringing her out" a little..Edmund becomes aware I think of his cousin's isolation and he encourages hte visit to Sotherton and then when Sir T come home and is warmer and more affectionate than before, he has the ball.
I think that the lakc of balls for hte older girls may have soemthing to do with finances but I dont really htink that hte Bertrams are that badly off, otherwise how come a couple of years later Sir T is able to fund a ball for Fanny? Would things have improved that much in 2 years if they were really bad or have I got the timelines wrong? I think its more to do with Lady B's indolence and her letting her daughters just go to local assemblies etc as their come out...
but I think that teh Bs have just overlooked any need that F might have for more compnay or for some kind of social life which might help her find a husband oout of carelessness...