The Collaborative International Dictionary of English v.0.48:

Puerility \Pu`er*il"i*ty\, n.; pl. Puerilities. [L.
   puerilitas: cf. F. pu['e]rilit['e].]
   1. The quality of being puerile; childishness; puerileness.
      --Sir T. Browne.
      [1913 Webster]

   2. That which is puerile or childish; especially, an
      expression which is flat, insipid, or silly.
      [1913 Webster]

WordNet (r) 3.0 (2006):

puerility
    n 1: the state of a child between infancy and adolescence [syn:
         childhood, puerility]
    2: a property characteristic of a child [syn: childishness,
       puerility]