From an imdb review:
Pitt plays the character at almost every age, but it’s almost impossible to tell when the CGI is being used on him. You know it’s there, obviously, but you can’t tell it’s being used. When the transition is just smooth enough for the Visual Effects to be retired, but just rough enough to use makeup, it’s absolutely perfect. If you’ve ever wanted to see Brad Pitt look 20 again, look no further, as the effects that make our actors young again (the same goes for Blanchett) are just as stunning as those that make them older.
Thing is with screen big enough and a 1080p source – you can tell the difference between Brad Pitt and CGI. And you can do it quite easily. Both old and young Benjamin look very much CGI alike. And it’s not that the effects are bad, because they’re not, it’s just that at this level of details (and face close-ups) you need to have your effect to be close to perfect. Seems we’re not quite there yet. The skin, the wrinkles, small hair covering the face, freckles, moles, every face feature – those are things you see and appriciate the most when viewing good HD content. And they make the biggest impresion. I know they did on me. They say animating a face is the hardest thing to do. At HD it’s ten times harder.