This is explained quickly. 年 means year and 青 stands for the color(s) blue/green. This is the color of plants that are still alive and thriving, still fresh. Thus 青年, literally blue/green years, is used to refer to persons who are in the best years, around 20 years of age.

Of more interest is the color 青. Color is light of a particular frequency out of a continuous spectrum, ie this no absolute border between two adjacent colours. Think of a black-white gradient:

Can you tell exactly where the border between white and gray is? Black-White Gradient

Keeping this in mind, it shouldn’t be surprising that different cultures would divide the color spectrum into different colours. And in Japan, 青 ao stands for a colour that is somewhere around blue/green. It should also not be surprising that certain types of green and blue, which we call by different names, may both be termed ao in Japanese.

PS: The traffic lights in Japan contain the colour ao.

PPS: Actually, ao was originally a vague, generic terms which meant almost nothing:

明鏡国語辞典
あお【青(▼蒼・▼碧)】 アヲ
[…]
→古く、赤を明るい感じの、青を暗い感じの色合いに使ったことから。

and we also find that people do not care about western conventions in their daily life:

精選版日本語国語辞典
あお あを【青】
㊀〔名〕
① […]本来は黒と白との仲間の範囲を示す広い色名で、主に青,緑,藍をさし、時には,黒,白をもさした。
[…]
[語誌]
(1)アカ・クロ・シロと並び,日本語の基本的色彩語であり、上代から色名として用いられた。アヲの示す色相は広く,青,緑,紫,さらに黒・白・灰色も含んだ。特にミドリとは重なる部分が多く,「観智院本名義抄」の「碧・緑・翠」には「アヲシ」「ミドリ」などの訓[読]が見える。
(2)色名としてのアヲは,ミドリ(これも若やいだ状態を表す意が早い)が緑色(グリーン)の色名として定着するにつれ,狭く青色(ブルー)を示すようになるが,なお,ブルー以外の色にも使われ続けている