The demon slayer corps is a big group of people who fight against demons, demons can only come out at night and can be very dangerous, the highest rank of the demon slayer are the hashira. You need to finish training with a master to take part in the demon slayer corps but Tanjiro inoske and zenitsu can't train you. All of the demon slayers are around 14-20. Zenitsu, inoske and Tanjiro are all 5'5 and nezuko is 5' tall. Training can be started from 13 years old but can take years to complete.