I'm going to cast my answer in terms of the badness of death, rather than the value of life. I believe that this does not affect the logic, one being the mirror of the other. Sections 16 through 21 of the Yale online course on Death are probably relevant in this sense.
There's a reductio ad abusurdum to indicate that we can't consider all potential intelligent life as equally valuable: there are an incomprehendably large number of "potential people" if you consider all possible egg+sperm combinations between all differently sexed pairs of people (even more if you go out a generation or two). Nobody worries about the lost value in that these potential intelligences never came to fruition (or at least the "value" of any one of these needs to be incomprehensibly small). We need to draw some sort of line, or have some sort of weighting to balance the "value" of these potentials; and that threshold/weighting defines where in the progress from a gleam in your fathers eye to being a fully matured person that we're going to allocate value.
Thomas Nagel's paper Death provides arguments that assessments of the badness of death as a form of deprivation need to take into account the past history of the individual in question (he's primarily concerned with people who have become fully intelligent). He argues that (part of?)the badness of death comes from the elimination of possibilities that the person would have had available but no longer does, and these possibilities depend on the entire history of that individual. In my view, this is just another argument that some threshold/weighting needs to be considered but doesn't provide much concrete guidance on where/how to make the distinctions.
I'd be remiss if I didn't include the standard, uncontroversial, observation that one person's death can be bad for his/her survivors, through their grief at the loss. For me, this would include even the very earliest stages of the development process: the anguish felt by some people who wish to bear children but cannot, is clearly a bad outcome, even though barely any steps have been made in the progression.
I thought that there was somewhere in the Yale course lectures (link above) where Dr. Kagan referred to a theory where the deprivation due to death was due to the fact that humans think about the future. (I haven't been able to locate it.) For animals that don't have this ability, it would be moral to kill (and eat) them, since they are not actually deprived by death (I'm bringing this up since the context of the theory might have been in justifying eating meat, rather than discussions of death). Anyway, if this idea, that death is only a deprivation to individuals that can, or to the extent that they can, form ideas about the future, (a) exists in the philosophical literature, and (b) is coherent, this would provide one possible specific feature on which to base the value judgments. Even if I'm mis-remembering the idea that an individual must possess one or more mental faculties in order to realize a deprivation from death may be an approach to pursue.
Another relevant mental faculty is the ability to feel pain; at the very least it's pretty generally accepted that we should avoid inflicting gratuitous pain on entities that can experience it. So it is almost certain that answering this would need to account for multiple features and their potential interactions in terms of "overall value".
To summarize: The idea that all potential intelligences are equally valuable seems hard to swallow, so some additional details need to be filled in there. Looking at the problem of "why is death bad" may be a useful approach since this question has been looked at in the philosophical literature (see the Nagel paper and references to/from it).