Preventative De-Worming ...what age can u start?

Hello. I am putting my girls out in the coop and run this week. They just turned 5 weeks old. I am hoping to free-range them after about 2 weeks in the coop and run for them to get adjusted to their new home. The question i have is...what age can you start preventative de-worming in RIR. Y wormer of choice will be "Valbazen" and to switch it up with another like "Safeguard" or another. I know some people don't do preventative and some do. I personally am going to choose to do preventative. We live in a forest and with them being free-range I am sure they will pick up all sorts of worms. I just have no idea what age you can start it. I already plan to follow up with Pro-biotics and such after the doses. I also plan on doing it twice a year, Spring and Fall. If anyone has any info. on the age of the chickens please let me know. Thanks.