CGI is an amazing tool.................but it's not a miracle worker.
De-aging CGI still seems to me to be very variable (and costly). To date, I think the de-aging work done on Stewart and McKellen on X-Men: The Last Stand was better than the de-aging work on Bridges in Tron: Legacy, which I have to say for me was (on it's own, not the whole film) one of the more terribly unconvincing visual effects I had seen in a major movie in a long time.
'If the Apocalypse starts, beep me!' - Buffy Summers
'The sky's the limit.....' Jean-Luc Picard, 'All Good Things'
courtesy of Saquist