I doubt any older singer who shows signs of not consistently singing high notes in tune is having problems with his hearing. It is the whole vocal mechanism that has changed.
It is most common for the vocal folds to thicken somewhat and to also lose some of their flexibility with age. This does not necessarily mean that the singer is no longer able to sing the required high notes but that the basic sound produced by these slightly thicker and less elastic vocal folds is different. The vocal quality up high will change.
And the singer enhances the sound produced by the vocal folds with adjustments of the vocal tract which includes all of the space above the vocal folds through the pharynx, mouth and even to the facial lips. It does not include the sinuses or the nasal passages. The major adjustable element in the vocal tract is the tongue but adjustments are also made by opening and closing the jaw and widening the pharynx and even some lifting of the soft palate. In effect, the singer is tuning the resonance potential of the vocal tract to match the pitch and this greatly increases volume of sound and the gives control over the tone quality of the voice
All these adjustment are learned while the vocal folds are still young and the sound they produce is consistent year after year. When the vocal folds age and the sound they produce is changed it requires new changes in the vocal tract adjustments if the resultant tone is to be maintained as similar as possible to that produced by the voice when it was younger. In effect the singer must replace previous vocal tract adjustments with new ones that will better serve the changed vocal fold tone.
However, old habits that worked so well are difficult for the singer to change even if he/she is aware that the solution to the problem is new adjustments of the vocal tract. The most common self correction singers use is to sing with more tightly closed vocal folds and increase breath pressure to push the air through the resultant vocal fold stiffness. This is exactly the wrong answer; the tighter closure of the vocal folds will slow down their rate of oscillation and this will, of course, cause the pitch to be flat.
Without making changes in vocal tract adjustment the older singer will always be looking for some muscular way to change the singing and this will only make matters worse. Some singers are so well aware of how they control the voice that they will instinctively make new vocal tract adjustments as the voice ages. But these are the rare few.