CREATE TABLE table_name_20 (name VARCHAR(255), mission VARCHAR(255), age_at_first_step VARCHAR(255))
Which astronaut went on the Apollo 16 mission at the age of 36y 6m 18d to step on the moon?
    
SQL: SELECT name FROM table_name_20 WHERE mission = "apollo 16" AND age_at_first_step = "36y 6m 18d"