I think if an adult did not have to work, placate a S/O, and nvagate his own way thru life, adults would learn tennis as fast as any kid.
School is not that hard, I'm sorry to say. Some people make it harder than other's, but you gotta consider your overall goal. If getting straight A's is necessary, then your priority is there, and not in tennis.
As a 3rd year player, at age 27, my practice partners were No1's and 2's of MissionHIgh, CityCollege of SF, and the top local juniors around SanFrancisco.
They considered me good enough to set regular practice dates with, and most became my doubles partners for at least a few tourneys.
A couple went on to WIN pro WTA tourneys, while they were still my regular monthly practice partners.