以上 / 以下 (and above / and below)

Eighteen and over, thirty and under

• that level or higher以上yǐ shàng = "and above / over"; that level or lower以下yǐ xià = "and below / under".
• 十八岁shí bā suìthat level or higher以上yǐ shàng = "18 and over"; thirty三十sān shíperson人rénthat level or lower以下yǐ xià = "fewer than 30 people".
• They attach after a number or level.

Number / Level + that level or higher以上yǐ shàng / that level or lower以下yǐ xià

• that level or higher以上yǐ shàng / that level or lower以下yǐ xià set an upper or lower boundary.

eighteen十八shí bāyears岁suìand above以上yǐ shàngonly then才cáican能néngwatch看kàn。

• 十八岁shí bā suìthat level or higher以上yǐ shàngtalent才能cái néngto see看kàn = "only 18 and over may watch."

Recap

Read the full reference: HSK 5 grammar — Prepositional phrases.