Early conversations about sex send the message that sex and sexuality are healthy parts of life. It’s important for children to feel OK about coming to you for reliable, honest information. Explain things at a level children can understand and make sure they have the facts.
I'm a firm supporter of teaching children from the start correct names for body parts. Not only does this helps to send the message that talking about these parts of our bodies is healthy and OK, but it's also important in setting your child up to protect themselves.
This then feeds into talking about and teaching consent and boundaries (2 to 5). 6 to 8 becomes the range to begin more detailed talks around what sex is, including puberty (how bodies change as we grow), and sexual abuse (no one can touch them without their permission).
By 11, you want to start having conversations about sexual choices and safer sex, normalise safe sex. Talk about the risks and also protections. You should also be discussing digital rules and values, not just around porn but also sending or posting explicit photos. The world has changed, some kids are sexually active from a young age, be prepared.
If sex and sexuality is normalised in your family, the teen years should be much easier with teen willing to open up to you. However, it's important to keep the messaging clear. It's more detailed now, particularly on topics such as assault/date rape/unwanted pregnancy/STIs.
At the end of the day, sex/sexuality is simple biology and part of life. By being embarrassed or "protecting" kids from knowing the truth, not only are you holding them back but also putting them at risk of making unsafe choices or even assault/abuse. Sex isn't a once off talk, but rather an ongoing discussion tweaked to your kid's development as they age. But there are certain ages which you should be hitting for education.