Unconscious Bias
This is a great inside look at how unconscious bias works: A boss pretends to be his female subordinate via email, and he’s shocked by how horribly he’s treated.
The bias here could be because the subordinate is female, because she’s younger, or because she’s a subordinate. All can be factors.
How are you judging your employee’s “performance” – based on what you witness or on what is reported to you? Those could be very different evaluations. Make sure you know what’s actually happening before you criticize your people’s work.