suffix
under ... years of age; less than ... years old
Suffix attached to a number indicating an age threshold; used in formal or legal contexts to specify age limits.
このアトラクションは6歳未満の子どもは利用できません。
Children under 6 years of age cannot use this attraction.
18歳未満の方は保護者の同伴が必要です。
Those under 18 years of age must be accompanied by a guardian.
以下 means '... or below' and includes the stated number, while 未満 means 'under' and excludes it. For age, 18歳未満 means under 18, whereas 18歳以下 means 18 or under.
Compound of 歳 (sai, 'years of age') and 未満 (miman, 'less than, under').