Certain words may appear to be ‘disyllabic adjective + noun’ without 的de after the adjective. This is because they are considered to be one word, not two words. Examples:
nián qīng rén
youth; young person
gǔ diǎn yīn lè
liú háng gē qǔ
 Yip, P. C., Rimmington, D., Xiaoming, Z., & Henson, R. (2009). Basic Chinese: a grammar and workbook. Taylor & Francis.