Researching Medicine Technology increases the age at which villagers become elderly.

There are a fixed amount of crops on the farm each time they appear, and they are replenished on a fixed schedule. Putting more farmers there will cause the farm to be empty longer, since all of the crops are harvested much more quickly with many farmers gathering them. You aren't being shortchanged on your crops, since the next cycle of crops will appear when they are scheduled to grow, based on game speed.