I believe parents need to be comfortable first, with saying the proper words in relation to sexual education. Example, refrain from giving "nicknames" to identify body parts, and instead, to stick to its scientific name for better understanding and clarity. This could open doors of genuine curiosity rather than children withholding their thoughts and assumptions. Parents could also assess what is the level of child's readiness and understanding to discuss this topic by asking them first "what do you know and what do you want to know" , and it could gradually lead from there.
I believe there is never a specific age to start having conversations about this, especially considering the kind of exposure our kids have :)