Hello. I have no idea about the over-18 thing but I seriously doubt it. If there is an age limit it would probably be somewhere around the 12 and over mark.
Secondly, in a city like Chicago, I would say it would sell out quickly but maybe not straight away. I wouldn't panic-buy but don't hang around til the day before either.
Thirdly, technically, you're there to listen to the music not to watch anything so obstructive viewing shouldn't really bother you. If you really want to see the orchestra then it might be a problem, however, as long as you can hear it (which you will be able to, folks sit quietly during symphonies) you should be fine.
Finally, I have never been to the Zelda symphony so I don't know what the average dress is but I have been to classical concerts in general and you should dress pretty smartly- maybe white tie. However, remember you will be sitting there for a couple of hours so don't wear anything ridiculously uncomfortable.