Jeff Bridges stars in both films, young and old. In Tron: Legacy, they used CGI to model his face to seem younger so he could appear besides himself. Similar to the effects in Benjamin Button, Bridges face was mapped and digitally recreated, then stuck on a younger actors body. I think this is also what they used in The Social Network to create the Winklevoss twins.
This article shows how CGI has advanced through films from Tron to Tron: Legacy.