We are so careful to make sure that our daughters are ready for their first period. But after they start having regular monthly periods, they have likely reached the end of puberty and have entered sexual maturity.
When helping your daughter understand how puberty is different for a boy, it is helpful to show common pathways. For example, eggs are produced in the ovaries and sperm are produced in the testicles. They both leave and follow along inside a tube to get to their destination.
If you have adolescents who are 15+ years old, they have likely reached sexual maturity. Now it is time to keep the teaching going, but at a new level. This age group is often more challenging to get their support for your teaching attempts, but the topics will definitely be of interest to them.