I'm not sure if the Watchtower ever held a Young-Earth view.... at least not as young as the plain reading of Genesis.
My earliest recollection of the WT teaching on this subject was the 7 x 7000 years theory for a total of 49,000 years. The theory was that each creative day was 7000 years long. I'm not sure where that came from. But, I remember that being abandoned in the late 70's to early 80's. That's when the "millions of Years" were quietly adopted. I remember others more "advanced" than I in the organization explaining that we didn't know how long each creative day was. They adopted the concept that a 'day' could have been just a figure of speech, and not literal. However, that never really seemed like a fair description of the plain-reading of Genesis to me.
The Gap Theory is a Christian taught theory that allows for millions of years, prior to the creation of man. It pre-dates Darwin:
The Young Earth Creationists would raise these problems with the Gap Theorists though.
Some things look old. But, many things look young and are just simply out of place and don't fit in the standard naturalistic paradigm being sold to the public.