My understanding is, he stuttered through his childhood (and maybe 20s? Someone can fact check me). In his later years, before his presidency, the majority of articles had Biden citing that he had overcome his stutter. It wasn't until about half way through the campaign that he started calling himself a stutterer again. My 2 cents, if it's even worth 2 cents, is that he certainly had disfluency even when he would say he stuttered in the past, but for whatever reason he did not want to associate himself as still enduring a stutter.