person older than oneself (with more life experience)
Used to refer to someone older who can offer guidance based on their greater life experience, not just age. Often carries a respectful tone.
I always learn something from the stories of those older and wiser than me.
He is ten years older than me and has more life experience.
先輩 is a general term for a senior in a school, workplace, or organization. 人生の先輩 specifically emphasizes life experience and age, not institutional seniority.
Compound of 人生 (life) + の (possessive particle) + 先輩 (senior). The phrase literally means 'senior in life' and is a natural extension of the 先輩 concept to general life experience.