N4
いか
Less than (or equal to), Under・(and) below, Any less・fewer, Not exceeding
17歳以下の方は保護者の方と来てください。
For those of the ages 17 and under, please come with a guardian.
Explanation
「以下」, read 「いか」, after an amount means 'or less', and it includes that number itself.
「20歳以下」 includes age 20. 「これ以下」 and 「それ以下」 mean 'less than this/that'.
Seen in discount conditions, age limits and grades.
Mixed up with
「20歳以下」 includes 20. 「20歳未満」 does not; it means 19 or under. Often tested.
「以下」 can also say something is poor, as in 「期待以下」 (below expectations).