I'm guessing that it's best to do it when they themselves start asking questions. If they don't start asking by themselves, compulsory sex ed at round 12 will have to fill in. Sure, some may not be ready just yet, but they have to know sometimes.
There are some things they need to avoid doing though.
For example, using a huge, a little over 1ft long wooden dildo to demonstrate how to apply a condom, like during my younger sisters sex ed. Yes, ingraining into young boys a feeling of woeful inadequacy is a sublime idea. As is providing young girls with the idea the average man comes equipped with a girl-splitter.