The important word there is "tend". For most people their personality slowly shifts over decades. However, that doesn't mean faster change is impossible. People's personalities can change quickly, either due to life changing events or to intentional effort to change - for example, nearly anyone who is neurotic, depressed, anxious, easily angered, or similar who practices mindfulness daily for 6 months will experience a reduction in those aspects of their personality. Combine that with CBT, new hobbies, changing to a less stressful career, choosing better friends, more exercise, better diet, better sleep, and so on, and you'll experience even faster change. However, most people don't do this, especially when they are older, and hence the statistics show that changes tend to be slow.