The Galactic Government has existed since the Galaxy was first created: So

The Galactic Government has been here on earth since Earth was created to. So, Galactic government agents have always been here in some form too and a part of all governments on earth ongoing since before there were governments and only loose knit tribes of people wandering the earth. So, Trying to say that UFOs are a new thing when they pre-existed even mankind on earth is likely the best point of view. There has never been a time when they weren't here with us.