When is death worst for you? Here are a few possible accounts of the harm of death, and the answers each imply:
(1) Pure Deprivation Account: death is worse for you the more (of a good future) it deprives you of. The worst time to die is thus the moment you come into existence.
(2) Discounted Deprivation Account: since what matters in survival is psychological continuity, death is worse for you the more it deprives you of future stages that are relevantly psychologically similar to your present self. Given that young children are drastically different, psychologically, from the adults they grow into, they are not so deprived by the loss of their adult stages as an adolescent or young adult would be. Depending on the precise details, we may expect this account to typically imply that young adults are most harmed by death.
(3) Present Preference Account: death is worse for you the more of your present preferences (e.g. for future goods or achievements) or projects it will thwart. This may vary drastically from person to person, but again we can at least expect that adults will typically be more harmed by death than small children, on this account.
#1 seems implausible to me. An earlier death is not always more tragic, even supposing that it thereby deprives one of more good times. Intuitively, it is more tragic to come close to some great success, only to lose it all at the last moment, than to die before one even had a chance to begin such an ambitious project.
The latter two accounts accommodate this intuition by holding that the ultimate success of one's projects wouldn't be (much of) a benefit to one's younger self anyway. We effectively deny the younger self full ownership of their distant future. Since you cannot be deprived of what you do not "own" (in the relevant sense) in the first place, we obtain the result that the younger self is not so harmed by death as we might otherwise have thought.
One difficulty with this view is that it sits poorly with the commonsense view that disciplining a young child now may be good for him on net, if it will bring him to have a better life in the distant future. If distant benefits are not benefits to this child, then our justifications would instead have to be impersonal and utilitarian: we discipline this young child for the sake of someone else -- the adult he will grow into. This is almost as counter-intuitive as the implication of #1 that we sought to avoid. Is there no way to accommodate both intuitions: that later deaths may be more tragic, and yet also that later benefits should indeed count as (significant, not excessively discounted) benefits to the younger self?
I think there is a way. The previous strategy sought to establish mature deaths as more tragic by discounting the harm done (the benefits lost) by childhood death. But there is another way to go here. We could hold fixed the deprivational disvalue of childhood death, and instead argue that additional harms are suffered in some cases of mature death. Some kind of pluralist account, combining #1 and #3 in appropriate proportions, might accommodate both intuitions.
By allowing that the younger person fully owns their future, we can say that later benefits are also benefits to the younger person. And yet we are not forced to conclude that an earlier death, by depriving them of more, must thereby be worse for them. For in addition to the harms of deprivation, we also need to consider the harms of thwarted projects and preferences. Pluralists may hold, of someone who lives a bit longer and in this time starts some new ambitious projects, that while they have been deprived of fewer future experiences than they would have been in case of an earlier death, this comparative good is outweighed by the additional tragedy of having an active project fail. Hence we can make sense of how it is that this later death is more tragic (a greater harm to the individual it befalls) than an earlier death would have been.