conditions = [
    (df["age"].lt(10)),
    (df["age"].ge(10) & df["age"].lt(20)),
    (df["age"].ge(20) & df["age"].lt(30)),
    (df["age"].ge(30) & df["age"].lt(50)),
    (df["age"].ge(50)),
]
choices = ["baby", "kid", "young", "mature", "grandpa"]

df["elderly"] = np.select(conditions, choices)

# Results in:
#      name  age  preTestScore  postTestScore  elderly
#  0  Jason   42             4             25   mature
#  1  Molly   52            24             94  grandpa
#  2   Tina   36            31             57   mature
#  3   Jake   24             2             62    young
#  4    Amy   73             3             70  grandpa