I wouldn't tell them why unless they specifically ask. Then I would tell them as gently as possible. Explain to them that you are trying to attract non-trad folks and need the strongest, in-their-prime voices available. Blame it on the audience - short-attention span, unfamiliarity with folk, one chance to catch them in the *folk-net*, etc. (JUST suggestions!)

However, I agree with the others about voices getting better, too. That said, I remember when my dad at around 85 or so lost his ability to hit in the higher registers and just didn't sound as good anymore. Never did figure out a way to tell him and it didn't seem to matter. His audiences were mostly older than him and didn't seem to notice or mind.