以下【いか】

以下 after a number means "or less", "and under". The number itself is included: じゅう はっさい１８歳いか以下 is 18 and under. It goes straight after the number and counter, and takes の before another noun.

以下 or 未満

みまん未満 is "under" and leaves the number out. At exactly 18 you are inside 18歳以下 but outside 18歳未満. The upward partner of 以下 is いじょう以上, which also includes the number.

Below a level

After a word for a standard, such as へいきん平均 (average), 以下 means "below" it, and it usually sounds like a poor result.

"The following"

In writing, いか以下の points ahead to a list or a text that comes next.