Can a plan "age" reach zero? I am reading SQL Server Execution plans by Grant Fritchey, He mentions: 


  SQL Server does not keep execution plans in memory forever. They are
  slowly aged out of the system using an "age" formula that multiplies
  the estimated cost of the plan by the number of times it has been
  used. The lazywriter process, an internal process that works to free
  all types of cache (including the plan cache), periodically scans the
  objects in the cache and decreases this value by one each time.
  
  If the following criteria are met, the plan is removed from memory:
  
  
  more memory is required by the system 
  the "age" of the plan has reached zero
  the plan isn't currently being referenced by an existing connection.
  


He also mentions earlier in the book the following: 


  Once the optimizer arrives at an execution plan, the estimated plan is
  created and stored in a memory space known as the plan cache –
  although this is all different if a plan already exists in cache.


I would assume that the plan could theoretically reach zero if the actual and estimated plan differ. This would give the estimated plan execution count zero even though it was stored in the cache. 

My question is What are the different scenarios that a plan age could reach zero? And am I correct in my assumption?

Fritchey, G. (2012). SQL Server Execution Plans. Springfield, USA: Simple Talk Publishing.
My question is when could the Age of a plan reach zero?

The algorithm that SQL Server uses to determine when and how plans should be removed from cache is called the eviction policy.
The cost of plan is analyzed to determine which plans gets evicted. Upon detecting memory pressure, zero cost plans are removed from the cache and the cost of all other plans is reduced by half.

For adhoc plans, the cost is considered to be zero, but it is increased by one every time the plan is reused.
For other types of plans, the cost is a measure of the resources required to produce the plan. When one of these plans is reused, the cost is reset to the original cost.
For non–adhoc queries, the cost is measured in units called ticks, with a maximum of 31. The cost is based on three factors: I/O, context switches, and memory. Each has its own maximum within the 31-tick total.


When not under memory pressure, costs are not decreased until the total size of all plans cached reaches 50 percent of the buffer pool size. At that point, the next plan access will decrement the cost in ticks of all plans by 1.
Once memory pressure is encountered, then SQL Server will start a dedicated resource monitor thread to decrement the cost of either plan objects in one particular cache (for local pressure) or all plan cache objects (for global pressure).

so the gist is ...
The Clock algorithm sweeps the cache at regular intervals. Every time an unused entry is found, the cost is decreased by some amount. If the cost is 0 and is not used, then it is removed from the cache.
Best references :

Plan Cache Internals
An in-depth look at SQL Server Memory–Part 1 and Part 2