少年可知曾经我也单纯如纸
[shăo nián kĕ zhī céng jīng wŏ yĕ dān chún rú zhĭ]
It literally means, 'Teenager, do you know I was once naive just like you'. Through this phrase, it conveys the feeling of nostalgia over the purity and simplicity lost during growth, perhaps also hoping young people cherish their precious times.