From Bulbapedia, the community-driven Pokémon encyclopedia.
May-December Couple is a type of Trainer that first debuted in Generation IV. So far, they have only appeared in Pokémon Battle Revolution. These Trainers use the Ice- and Grass-types in reference to the spring and winter seasons.
The trainer class name may also be a reference to a "May-December Relationship" which refers to a couple in which one partner is significantly older than the other.