Hi @JurIvk There are two difficulties with what you are wanting to do.
Avoid using system fields directly in equations. The ‘Created At’ field is system generated, and fields like ‘Created At’ or ‘Record ID’ are unusable in formulas. It’s better to create your own Date/Time field, such as ‘EnteredOn’, and set up a record rule to insert the current date and time when the record is created. If you already have records but no such field, you can also add a record rule to copy the value from ‘Created At’ into ‘EnteredOn’, however you then hit the second problem…
Equation recalculation behaviour. Your ‘Task Age’ equation field containing the equation: DATEDIFF(NOW(), {EnteredOn}) will only update when the record is modified. This means the task age value will stay the same until the record is edited or an automation updates it.