When Melkor is condemned to the prison of Mandos, it is for three ages.
There was Melkor doomed to abide for three ages long, before his cause should be tried anew, or he should plead again for pardon.
And
For it came to pass that Melkor, as the Valar had decreed, completed the term of his bondage, dwelling for three ages in the duress of Mandos, alone.
Are there any canonical references to the start and end points of the time periods called ages, during the Years of the Trees?