Well, The Daley (since 1977) Civic Center was built in 1965. So I would have guessed that year. However, From it's old location at Congress, East of Michigan Av. It moved a year later.
The Civic Center/Daley Center 'tree' used to be a much larger assemblage of smaller trees. I have some old photos of the 'tree' from the late '70s, showing it much larger. I'm not certain when they switched to the individual tree.