The fxguide‘s X-Men Extreme Makeover explains how the actors playing Professor X and Magneto were made to look years younger:
These anti-aging shots are not achieved by the use of custom software or sparks, rather the team at Lola relied on standard inferno and flame software. But while the programs are off the shelf, the techniques have been developed over several years. The team cleverly uses a huge combination of discreet tools such as extended bicubic patches meticulously tracked in a technique the team calls “digital skin grafts”. But every trick in the book is used, from advanced colour correct to 3D tracking inside action.The anti-aging of each actor is built on extensive research, both visual reference, and medical advice. Lola consulted with leading hollywood plastic surgeons and a vast library of material. It was discovered that noses continue to grow, as to ears, so noses were thinned and ears were removed resized 10% and retracked back on.
While Greg Strause was on set for much of the filming of this sequence, by his own admission, there was little to do. The actors had no special makeup, not even hair colouring. There were no special tracking markers, greenscreens, measurements or considerations given to the effects team.
The sequence was initially scheduled to be a makeup effects, when tests were not satisfactory, alternative actors were briefly considered but thanks to a previous relationship with Visual Effects Supervisor John Bruno, Lola was given the chance to helm the sequence. Lola’s first test was 19th of Nov 05. Not only did Lola contribute the opening sequence but the company was also used on “hundreds of other little” shots according to Bruno.
Definitely compare the before and after photos.