I want to say that Bucky must have known that Steve was going to diverge from the plan and stay in the past because he said "I'm gonna miss you". Now from what Bruce said it would only be 5 secs of their time so there should be no reason for Buck to say that to Steve if he was gonna come back the same Steve. Also, I think he and Steve discussed who the shield would be going to which is why he encouraged Sam to go first. Bucky already knew Steve would be old.