When a story spans decades, it is often not possible for the one actor to play the same character throughout. Heavy make-up might just let the 30 year old play 90, but never 10. Instead, multiple actors are used for the same role.
A Timeshifted Actor will usually be credited as playing 'Young Mr X' or 'Old Ms Y'.
Differentiated from the other actor replacement tropes by the passage of long [...] [tvtropes.org]