Do tennis players need to reach a certain ranking to be able to turn pro?

I was looking at the ATP rankings and years the players turned pro and I couldn’t understand what is the criteria for players to be able to turn pro? I always thought that as soon player start to play ITF pro circuit and earn some prize money he is automatically is considered to be a pro, but I noticed with every single player on the tour that they had already 1-2 years of playing on ITF tour and having a certain ranking before they turned pro. Normally it was happening when they were achieving at least top 1000 ranking. Can someone please explain to me as I’m confused.