Older trees might begin bearing sooner, but they might not transplant as well. And keep in mind, larger trees are not necessarily older. Most bareroot fruit trees in the nursery trade have a one-year-old top (the fruiting variety) and a one- or two-year-old root system. Many variables influence whether a larger or smaller tree will better tolerate the transplanting and establish more quickly. It's entirely possible for a smaller tree to outgrow and outproduce the larger tree by the second or third year.
A containerized fruit tree, if well-grown and properly planted without appreciable transplant shock, might outproduce a new bareroot tree for the first year or two.
For the quickest possible significant crop, fall-planting a 2-3 year old tree in a 24" box is a good option, though expensive. Variety and rootstock selection of such trees is much more limited, but in some areas many of the proven popular varieties are available.
For more information, see Buying Fruit Trees.