I'm 25 and I'm a tattooed rock and roll guy, but I took my girlfriend to the philharmonic last month.
I wore a dress shirt, coat, and tie. She wore a nice dress.
FWIW I don't think it should matter how old you are or what generation you are, it's more of a respect for yourself, your date, and the institution.
But apparently I'm an elitist snob according to some people here...
Just for the record, I don't think you're an elitist snob. I think you dress up when you feel it is appropriate attire and by the sounds of it, you treat your girlfriend with respect and treat her as she deserves. I do respect your opinions.