懵懂别稳重
[mĕng dŏng bié wĕn zhòng]
'Farewell to innocence (without) becoming too heavy/staid'. It refers to bidding goodbye to the carefree days while warning oneself or reminding the surroundings not to become overly solemn due to the changes brought by age, environment or circumstances. In fact, there could still remain youthful lightness.