The point at which time dilation becomes significant for interstellar travelers depends on the speed at which they are traveling relative to a stationary observer and the duration of their journey.
According to the theory of special relativity, time dilation becomes noticeable at speeds approaching a significant fraction of the speed of light (approximately 299,792,458 meters per second). As an object approaches the speed of light, time dilation effects become increasingly pronounced.
For interstellar travelers, who might aim to reach significant fractions of the speed of light to traverse vast distances, time dilation would become significant during their journey. As their velocity increases, time dilation causes time to pass more slowly for them relative to a stationary observer, such as people on Earth.
The significance of time dilation would be directly proportional to the speed of the spacecraft. For instance, even at a velocity that is just a fraction of the speed of light, such as 10% or 50%, time dilation effects would still be noticeable, albeit less extreme than at speeds closer to the speed of light.
The duration of the journey is also a crucial factor. The longer the interstellar voyage, the more time dilation effects would accumulate. Even small fractions of time dilation can accumulate into significant differences over long periods, resulting in substantial differences in the aging of the travelers compared to stationary observers.
Therefore, for interstellar travelers, time dilation becomes increasingly significant as their velocity approaches the speed of light and as the duration of their journey increases. Both factors contribute to the observable effects of time dilation on their aging relative to those who remain stationary.