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.