How do we predict the lifespan of dinosaurs?
Your first question appears to be answered (tentatively, at least) in the article you linked to...


  "Traditional" estimates based on slow, reptilian growth rates,
  combined with the enormous size of dinosaurs, led scientists to
  conclude it could be up to several hundred years. However,
  palaeontologists today believe that dinosaurs grew much more quickly,
  rather like birds and mammals.


Then...


  Dinosaur bones grew like those of other vertebrates, by adding new
  bone matter to the outside of the bone. Because of annual variation in
  temperature or the availability of food, periodically bone growth
  would slow down, and a thin layer of a vascular bone would form a ring
  or "growth line" in much the same way that tree trunks do. By taking
  thin slices of bones, these rings can be viewed under a polarised
  light source. Counting the rings can give an idea of the dinosaur's
  age at death.


Theoretical ages of some specific species are given in various articles, including Wikipedia and LiveScience.

The infamous T. rex theoretically reached adulthood at the age of twenty (similar to humans) and reached an age of at least twenty-eight years. I think most contemporary animal species mature much more quickly than that...but few approach T. rex in size. The hypothetical ages given for some of the smaller dinosaurs appear to be more in line with modern living species.