noun
second time at life; reincarnation
Often used jokingly to describe precocious children who seem to have lived before, or in reincarnation contexts. The gloss 'reincarnation' is a direct meaning, while 'second time at life' captures the humorous usage.
あの子はまるで人生２周目みたいに落ち着いているね。
That kid is so composed, it's like he's on his second time at life.
人生２周目なら前世の記憶があるはずだ。
If it's a second life, you should have memories of your past life.
生まれ変わり is the standard term for reincarnation or a reincarnated person, while 人生２周目 is a more colloquial, often humorous expression.
Compound of 人生 (life) + ２周目 (second round). The expression likely originated in online or casual speech to humorously describe someone acting beyond their years.