person much older with much more life experience
A respectful, somewhat warm expression for someone significantly older who has lived through much more. Often used when seeking advice or acknowledging their wisdom.
I had a person much older and wiser listen to my story.
He is a great senior in life to me.
先輩 is a general term for a senior (in school, work, etc.), while 人生の大先輩 specifically emphasizes a large age gap and life experience.
Compound of 人生 (life) + の (possessive particle) + 大 (great) + 先輩 (senior). The expression is transparent and modern.