why is that their problem?

That's the fundamental question. So you started singing at 27; awesome! If you're a great singer at any age, then that's great. But because you started at 27 doesn't mean they need to change their request for "young" singers just because you started a little later than some other people. You may even be a better singer than those 25 year olds. But that's not the point. It shouldn't be a problem that you may be too old for whatever this person wants---that's just how it is.