Old dogs and their new tricks.

AS FURTHER PROOF that Hollywood excels at irony, the same week that older writers were awarded $70 million to dispense with an age-discrimination lawsuit, a comedian who'll soon turn 60 uprooted a guy 13 years his junior as host of "The Tonight Show."

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In some respects, NBC's decision to bet the farm on Jay Leno--even if that meant losing Conan O'Brien, who balked at a proposed compromise arrangement--does reflect a demographic watershed. Let's not forget that the network opted to elbow Leno out back in 2004, postponing the baton pass to an eager O'Brien for five years. It did this in part because it was anticipated that Leno couldn't keep …