"The talk" when and how

At what age should parents talk to a child about the birds and the bees?

And how should the topic be brought up?

I wish we started earlier in life, so we could build on the conversation. But we didn't.

Oldest son- October he'll be 13

Twins -son and daughter - 10 in Aug.

And new baby is 3 months.maybe I'll talk to him about it first. For practice haha

I know kids these days know a lot. (Frienda, school, etc) and I want them to know the right info and know what's important. and to be establishing values and morals.

It's just hard to bring up. I want them to be comfortable talking (and listening)

Any tips? Advice? Personal stories or experience!?