That's an interesting take on the idea. There may be, almost certainly is an "optimal" point of view where the balance of future carrying cost, productive potential, experience and future work expectancy.
If you value experience the highest, then older people are the most valuable. Children have highest carrying cost, least experience, but the highest adaptability and future earning potential.
Now you could take a *market* approach to valuing lives by holding an auction to see how much people will contribute to save a life. In that case I have no doubt that children would win hands down. In a sense we do this already; charities which rescue children have a distinct advantage over those that target adults or the elderly.