Had the talk with my boys in small chunks from 11-13 years old, giving a little more detail each time. At first they were more uncomfortable than i was....but it became more natural over time. Actually made them feel more comfortable asking questions about other things too as they got older.
This is really good. Think of it as an ongoing conversation. Practice with your spouse. Get used to saying penis, vagina, erection, masturbation, etc... It's awkward, but has to be done. For others, I'd advise beginning even sooner. Use the right names for "parts." Make sure you focus on the "bigger picture," what you believe about sexuality, etc... You want to create an environment where you can talk about anything. Yes, anything. If you do, it will help your relationship in almost every way.