I'm in a similar boat as yourself. I recently got my best friend into Vienna and was ecstatic when I saw her performing in NYC again this year (I saw her for the first time live at the Joe's Pub venue this year and was floored
...in a good way, of course
). I just turned 20 in March and my friend turns 21 in November. It's not the end of the world if we can't get in, we'll see her live together in the near future regardless, of course, but it would
be a nice addition to our summer vacation plans.
I'll probably contact Union Hall myself soon to inquire about the age limit, but let us know what they say about your situation when you find out!