Our current understanding of the plant family Ericaceae (which includes rhododendrons, blueberries and heaths) suggests that this genus, Enkianthus, is the “oldest”, if you are speaking in evolutionary terms.
In other words, if you investigated all of the genera of plants within this family, used shared characteristics to determine how closely plants were related, and then calculated the relationships between the genera based on those characteristics, you would discover that Enkianthus has been around the longest.
All other woody members of Ericaceae have characteristics which Enkianthus lacks; one of these is tetradinous pollen. This simply means that the fully-developed pollen grains are fused as a unit of four, and this is the case for nearly all woody Ericaceae except for Enkianthus. By contrast, Enkianthus has monadinous pollen–each mature pollen grain is a single unit. It could be argued that the evolution of tetradinous pollen in this family was one of the changes that allowed the woody plants of the family Ericaceae to diversify (it could also be argued otherwise, as there are other characteristics that are different between Enkianthus and the others–see Kron, K. A., Judd, W. S., Stevens, P. F., Crayn, D. M., Anderberg, A. A., Gadek, P. A., Quinn, C. J., Luteyn, J. L. Phylogenetic Classification of Ericaceae: Molecular and Morphological Evidence. The Botanical Review. 2002 68: 335-423. Perhaps if the development of fused pollen grains in woody Ericaceae hadn’t happened millions of years ago, you wouldn’t be consuming blueberry jam or cranberry juice today.