How old is Casiquiare canal?
This is a part answer, as pinning down the actual date that the canal formed is not well framed in the literature.  

According to the Texas A&amp;M University web-chapter The Vaupes Arch and Casiquiare Canal Barriers and Passages  (Winemiller and Willis, 2011), are quite vague about exactly pinning down the age of the canal, with


  Extensive alluvial sedimentation and channel 
  meandering provided subsequent opportunities for drainage 
  capture between the Orinoco and Negro headwaters to the 
  east of the Vaupes Arch, and at some point the Río Casiquiare 
  formed a connection between the upper Orinoco and upper 
  Negro rivers


and by using digital maps based on the topography suggest that


  the upper
  Casiquiare course may have been captured quite recently by
  the Pasiba–Siapa–lower Casiquiare drainage network as a result
  of river meandering on the peneplain.


The article The Casiquiare river acts as a corridor between
the Amazonas and Orinoco river basins: biogeographic
analysis of the genus Cichla (Willis et al. 2001) are more direct in stating that


  Unfortunately, the precise timing of the origin of the
  Casiquiare remains unknown


also stating that in studies of the DNA of species in either basin, there seems to be little dispersal between them through the canal.  Specifically noting that the


  Casiquiare River acts as a selective filter, where only
  some species are able to make the transition from
  stained, acidic water in the Negro River to unstained,
  neutral water in the upper Orinoco River. 


Winemiller et al. (2008) state in the article Fish assemblages of the Casiquiare River,
a corridor and zoogeographical filter for
dispersal between the Orinoco and
Amazon basins (2009), that


  Hydrologically, the
  Casiquiare represents a stream capture in progress, but there is
  some debate concerning its formation


Secondly, an answer to your question as to why the water does not stay in the Orinoco, it is a question of elevation, according to Winemiller and Willis, (2011), it is a case of flowing downstream from the western edge of the Guiana Shield, the direction governed by the elevations of the two ends of the bifurcation.


  At its origin at the
  bifurcation of the upper Orinoco, the Casiquiare
  is about 90 m wide and lies at an elevation of 120 meters
  above sea level (m-asl). At its mouth at the upper Rio Negro,
  the Casiquiare is over 500 m wide at an elevation
  of about 90 m-asl.


and passes through what Winemiller et al. (2008) describe as an alluvium filled floodplain, that coupled with the meandering and stream capture (mentioned above) has governed the course of the canal.

The Rio Negro being a major tributary of the Amazon River.

Lujan and Armbruster, authors of the chapter The Guiana Shield (2011) assert that


  In the 
  future, it is likely that a new drainage divide will form within 
  the Orinoco downstream of the Tama-Tama bifurcation, and 
  the current headwaters of the Orinoco will become entirely 
  adopted by the Amazon