生かされている comes from 生かす (to keep alive or let live) in the passive form, so it means “being kept alive” or “being sustained by someone.” In the song, the singer says he is living because of the other person, giving this phrase a powerful sense of gratitude, dependence, and emotional survival.
"13 Years Old Night" is push pull pot looking back at one day that split a childhood in two. The time stamp gives it away: "14時46分 いつもと違う鐘が鳴った" (At 2:46 p.m., a bell rang that was different from usual). In Japan, 14:46 is the minute the Great East Japan Earthquake struck on 11 March 2011, so the singer is telling us he was thirteen that afternoon. The chorus asks the listener straight out, "突然すべて失ったこと あなたはありますか" (Have you ever suddenly lost everything?), and answers for himself: "13歳の僕は受け入れるしかなかったし" (at thirteen, I had no choice but to accept it). He is honest about the empty comfort people offered, too: "無責任の大丈夫が やけに頭を駆け巡った" (an irresponsible "it's okay" kept racing round my head). 大丈夫 (daijoubu, "it's okay") is one of the first words learners pick up, and here it stings.
What pulls him through is one person's smile. "辛いけど あなたが笑うだけで 僕は救われたようだ" (It's hard, but just you smiling seemed to save me), and later the much heavier "あなたに生かされているの 泣き叫んだ" (I'm being kept alive by you, I screamed and cried). I like the small changes across the choruses: "受け入れるしかなかった" (had no choice but to accept) becomes "受け入れたんだよ" (I accepted it). There is also a nice idiom, "過去の記憶とにらめっこしても" (even when I stare down my memories of the past). にらめっこ is a kids' staring game where the first to laugh loses, which makes "表情ひとつ変わらない" (my face doesn't change at all) land harder. The song ends moving forward: "怖いならば一緒に笑っていこう" (if you're scared, let's keep laughing together), then "駆け出した 13歳の夜" (I started running, that night at thirteen).