Most of the answers here are not very coherent. Any reasonable discussion of this topic needs to get back to the fundamentals. The baseline analysis does NOT support the conventional wisdom that a long horizon investor should invest a larger fraction of her portfolio in risky assets because she has an opportunity to average returns over a longer period.

If you are surprised by this read the papers by Paul Samuelson and Bob Merton.

Samuelson , 1963, "Risk and uncertainty: the fallacy of the law of large numbers"

Samuelson, 1969, "Lifetime portfolio selection by dynamic stochastic programming"

Merton, 1968, "Lifetime portfolio selection under uncertainty"

The second and third papers are the key ones (just focus on the introduction and problem setup since the math is pretty advanced) and give the answer that people should hold a constant proportion of wealth in risk assets.

More recent research tries to reconcile these findings with the conventional wisdom. One way of doing this is to introduce labor income. See for example Bodie, Merton, Samuelson 1988. The upshot of this that all the comments here that babble on about having "more years to recoup losses and rebuild" and don't mention labor income / human capital as part of the equation are totally missing the point.