prime of life; noontide of life
A somewhat literary or poetic set phrase meaning the peak or most vigorous period of one's life. Often used in reflective or congratulatory contexts.
He is now in the prime of his life.
She fell seriously ill in the prime of her life.
真っ盛り alone means 'in full bloom' or 'at the height' and can be used for seasons, events, etc. 人生の真っ盛り specifically refers to the prime of human life.
A set phrase combining 人生 (life) with 真っ盛り (height/peak). The exact origin is uncertain, but it follows a common pattern of using 真っ盛り to indicate the peak of something.