It will vary with the individual rooster and the breed. Some hens can start laying around 16 weeks, but the average seems to be about 20 weeks. Some hens don't lay for a lot longer. So, for roosters to successfully do the deed, you can expect some to be effective at 16 weeks, most at 20, but some will take longer. And just because they are trying does not necessarily mean they are successful. There are flock dynamics to consider. An older hen may not let a young rooster fertilize her eggs, where she would let an older one. So the answer to your question is, similar to many questions about living animals, it varies and depends on the individuals involved.
Do you have to help them? No.
Hope this helps.