Female singers may remark that they have beautiful high notes and beautiful low notes, but don’t ask them about their middle voice.  Those are the pitches where there is the most fatigue due to the fact that is also where the speaking voice lies.  

As we age all of our muscles lose elasticity and the  muscles in the larynx are no exception.  

In order to deal with this factor, older female singers should use their chest voice Eb 4 and lower. Tones should also have the sensation of much head space which will enhance the round quality of the vowels.