Rashford and AWB examples are fluctuations in form, an inherent lack of consistency, and/or fit with the manager's tactics and what the squad needs. This is different than an age-related decline in quality when a player hits a point where their experience and knowledge of the game is no longer able to make up for the loss of athleticism.

Outside of significant injuries, it's rare that a good player hits their peak in their early-mid 20s and can never get back to that. A lot will reach their potential and stop developing but they don't go backwards. That's why it's so surprising and attracts attention when it does happen, e.g. Dele.