Well, if you want the perfect career (1lv - 1st season - 1 league cup) you must start the first day.
But if you start now, as the other said, you have an advantage taking some young players.
You can find many young free agents, good trainers and spending 1 or 2 T as my example below.
I remember myself when I started, I was afraid to hit free agents thinking that maybe they had problems with injuries or they were bad players thats why the other managers set them free ...LOL

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