If you play games, it's when you start having to downgrade to lower graphics, and struggling to get good FPS. For video editing, it's when programs start to take a while, lag, crash, etc. frequently. It honestly depends on what you're doing, and what causes issues when doing it. For games, the big thing to update is your graphics card, programs would be Processor & Ram. If the computer is 5 years old, chances are it's super out dated, even the best, top tier PC is medium tier after 5 years.