There is a very close connection between this question and an earlier one on biennial plants. The right time to reproduce is when one has accumulated the necessary resources for reproduction. It is a field in which many, many examples abound, of extremes and opposites in the adaptation of evolutionary strategies to needs and environments. For example, some insects, such as certain scale insects, have males that, once they emerge from the pupa, amount to flying gonads. They are large enough, and well enough equipped to fly enough, to impregnate a female if they are lucky enough. That is about all there is to them. It is an unusual male that lasts for more than a day or so of maturity.However, commonly it does take about as long to reach maturity as a female, or there might be no mature female available when the male emerges.
Males of some spiders, such as some species of Latrodectus, mate just once, and then actually swing around into the female's jaws to get eaten. It is their final contribution to parental investment. A small investment, but huge in the sense that they invest all that they have. The only reason that they do not simply mate at birth is that at that time the females are not really either; it is no good producing a male as nothing but a sperm carrier if it cannot survive to mate when mating becomes possible. Males of may such species squat around the females' webs, helping themselves to scraps of the females' kills till their moment of truth arrives.
What Mike says is correct as far as it goes. There is not much point to producing sperm if the offspring stands a poor chance of survival without the male's hunting, defence, etc. However, I do not agree that parental investment typically is the largest factor here, though it certainly is important in some species.I do not offhand know of any quantitative studies of the correlation between the age of male sexual maturity and male parental investment, but while certainly it is a penetrating thought, I doubt that it is any more important in practice than any of a number of other selective pressures affecting the size and age of mating males.
One such factor, as we saw in the example of Latrodectus and the scale insects, concerns the question of matching the maturity of the female. In many environments a small male would stand little chance of survival to mating age. So slowly maturing females tend to have large males. The operative word however, is "tend". There are whole classes of exceptions apart from some spiders and insects. Consider angler fish. At least one family of them have males that live on the females as tiny parasites (literally!) and live on them for years or possibly decades, as attached gonads and nothing much besides.
Orgel's law amounts to something like: "Natural selection is smarter than you are."
Be that as it may, the size, physical elaborateness, and longevity of males is, I suspect, more frequently controlled by the necessary strategies for mating than for parental investment. Consider various seals and sea lions for example: males seldom mate until they have spent several years growing into fighting machines, machines specialised for fighting each other rather than defending females and young from enemies of the species. Many other animals have variations on such a strategy -- lions, horses, deer, mountain sheep and goats; look around and find examples for yourself.
Elephants are an interesting study in their own right. So are killer whales.
In a different way, so are pollen bearing plants. The offspring of pollen comes pretty close to the idea of instant gonad production in infant organisms. The semantics of such questions becomes pretty slippery!
The sheer richness of such classes of study in evolution leaves me at something of a loss.
It certainly is more than I can discuss sensibly in a single reply of this type.
Sorry about that.
Jon
