In reply to MRDIVABITCH.

Yes, voice declines with age. At like age 60-70, not at age 30. Unless there's some other issue like drugs (Whitney) or medical problems such as nodules (Mariah). In Christina's case it has decline due to complete lack of singing technique. Even Mariah and Whitney as they entered their 30's were in better vocal shape Christina is in now. The human voice, if taken care of, doesn't hit its peak till the mid 30's for medium size voices and mid 40's for large voices.