Time period + に入る

A time period + に入る would literally mean "entering into that time period", and is often used to express that period beginning. It's most commonly used in the form [Period] に入り or [Period] に入って(から) meaning "since [time period] began."

インターネットの時代に入って、人々の買い物の仕方が変わった。
Since we entered the internet age, the way people do their shopping has changed.

梅雨に入ってから、晴れの日がまったくなくて湿度が高く、気持ちが悪い。
Since the rainy season hit, sunny days have completely disappeared and humidity is high - it's unpleasant.

60年代に入ってすぐベトナム戦争が始まり、15年も続いた。
In the 60s the Vietnam War immediately began, and it continued for 15 years.

彼は思春期に入ってから、親にすごく反抗するようになった。
Ever since he's reached adolescence, he's come to greatly disobey his parents.