"在年富力强时" can be translated into English as "during one's prime years" or "when one is in the prime of life".
Both expressions convey the idea of being in one's most vigorous and productive period, typically during youth or middle age. For example:
He accomplished a great deal during his prime years.
(他在年富力强时取得了许多成就。)
One should make the most of opportunities when one is in the prime of life.
(人应该在年富力强时充分利用机会。)