I was the youngest child in our family, so I tend to have youngest-child-like character.
I feel more comfortable when I am around people who are older than me. I can act more naturally.
ご質問ありがとうございます。
"I was the youngest child in our family,"＝「私は家族の中で末っ子でした、」
"so I tend to have youngest-child-like character."＝「なので、末っ子気質です。」
"I feel more comfortable when I am around people who are older than me."＝「自分よりも年上の人たちと一緒いると楽です。」
"I can act more naturally."＝「もっと自然に振舞えます。」
ご参考までに。
I feel more comfortable around older people since I'm the youngest in my family.
「末っ子気質のせいか年上の人達といるほうが自然に振る舞える」は英語で「I feel more comfortable around older people since I'm the youngest in my family.」と言います。「the youngest in my family」は他の言い方が沢山ありまして「the baby of my family」（赤ちゃんのような存在）も同じような表現です。
末っ子気質のせいか年上の人達といるほうが自然に振る舞える。
I feel more comfortable around older people since I'm the baby of my family.
アメリカ合衆国 
カナダ 
