Some immunocompetent cells will never encounter an antigen to which they can bind and therefore will never be called to service in our lifetime.

It is true that some immunocompetent cells will never encounter an antigen to which they can bind and therefore will never be called to service in our lifetime. 
In reference to lymphocytes, immunocompetence mean that a B cell or T cell is mature and can recognize antigens and allow a person to mount an immune response. T-cells express T cell receptors that recognize the antigen information associated with MHC molecules.