The topic of death is a delicate one and it is understandable many are uncomfortable to discuss it. However, one should contemplate on this question: which comes first, tomorrow or death? We can never know for sure, but death does not discriminate and neither does it wait. So when it comes to the topic of the best time to pre-plan, there is no better time than now when you are already thinking about it.
In fact, the common perception of waiting till we are older to do it may not be entirely correct. According to data from Knoema, the number of deaths in 2015 between ages of 25 and 69 numbered at 338.25 (thousand cases). In comparison, the number of deaths at the same year for those above the age of 70 numbered at 309.59 (thousand cases).
The conclusion that we can draw here is that it is never too early to pre-plan. The sooner you do it, the greater your peace of mind. Similarly to the question of who should pre-plan, as long as you are ready to explore the idea, then you can proceed to do so.