To be honest I was just in this situation. I look like a teenager and I am in my mid-twenties. Most people off the street as me when what grade I'm in. Anyway, I auditioned for a teenage role, was told that my audition was great but they went with someone younger. We got to see everyone audition so I know that I out sang and out acted the other girl, as did the others who auditioned, but they went the political route. It's not always about who has the most talent, no matter what your age. While it killed my chances from the start (because I knew all of them personally) had I been younger it still wouldn't have mattered. People have preconceived notions and some just can't get past them. I decided to opt out of doing the show (I didn't put that I would accept any role, so I was able to decline the one I was offered) and from what I've been told it was the best decision that I could have made. Plus I went back to school which I wouldn't have been able to do had I done the show.
To stay professional you just go in and give them your best audition. Be friendly. If you'll take any role say so, and then do it. If not politely decline. I said that I didn't feel that I could give 100% to the one offered and if they had someone else who could do it then I would rather they have the role. My problem was that they tried to patronize me and tell me that they needed a strong singer for the role, which was a flat out lie. They knew that what they did casting wise for the entire show was political and that I refuse to be a part of shows that are cast that way, even if I benefit from it. Do what is best for you. Do you love theatre or the show enough that you'll enjoy being a part of this production in any capacity? If not don't do it. You'll make yourself and everyone else around you miserable.
There are directors that are open to having their minds changed based on how you audition. I once landed a role that was set for 30-35 at the age of 23 (remember I look even younger) just because I was the only one that gave the director what she was looking for. A friend of mine was able to change this same director's mind when it came to body type for a character. She is tall and overwight, the director wanted someone petit and thin, but again she was able to look past the physical and choose the talent. My friend even won a best supporting actress award for the role! Don't give up yet! You never know what will happen.
Currently~ Miss Myrna Thorn in Ruthless! the Musical
Recently~ Emma Carew (Jekyll & Hyde), Sister Mary Leo (Nunsense), Demeter (Cats), Tzeitel (Fiddler on the Roof)