You can retire at anytime.
Advanced notice has some benefits as the article mentions -
Many American pilots retired early to take advantage of a contracting quirk that enabled them to turn back the clock as they collected their retirement payout.
The advance notice doesn't apply post 60 so they've created a situation where they won't be able to figure out when a 60+ pilot will retire.
There is no legal obligation to notify the company when you plan on retiring.
If its true I find it pretty incredulous that an airline can lose such 'key' people with no prior notification except when they don't check in to their flight to Bejing (Employment contracts ?)
Do the so called pilots not even have the self responsibilty and good manners to notify the airline just for the sake of the passengers who have effectively paid their wages over the past 30 years
