in one's 50'sが決まり文句です。
応用問題です、20代前半に・・・は、in one's early 20'sです。
では、in one's dayはどういう意味でしょうか? これは若かったころ・・という意味です。
Driving at great speed under the influence of alcohol, the 32-year-old man collided head-on with a car moving in the opposite direction, leaving a couple in their 50s and seven others injured.