老成持重

tā lǎo chéng chí zhòng bù suí biàn fā biǎo yì jiàn

他老成持重，不随便发表意见。

He was experienced and reserved, and did not give his views on anything without due thought.

tā lǎo shì jīng shén dǒu sǒu ér yòu lǎo chéng chí zhòng xīn dì shàn liáng ér yòu gàn liàn zì fù

他老是精神抖擞而又老成持重，心地善良而又干练、自负。

He always looked jaunty and yet reserved, good-natured, and yet capable and self-sufficient.

bié zhuāng de lǎo chéng chí zhòng zuò nǐ zì jǐ jiù xíng

别装得老成持重，做你自己就行。

Don't act sophisticated. Just be yourself.