Translation

人生の全盛期 / 働き盛り / 脂が乗っている時期

Meaning

The period of peak physical and mental vigor, typically occurring during middle adulthood.

Examples

• She felt she was in the prime of life, balancing a successful career with a vibrant social calendar.
  She felt she was in the prime of life, balancing a successful career with a vibrant social calendar.
• Many athletes retire before they reach the prime of life, but he continued to compete well into his forties.
  Many athletes retire before they reach the prime of life, but he continued to compete well into his forties.