When should parents start talking about sex with kids? （家长应该什么时候开始和孩子谈性？）

As soon as they start asking . If a kid is old enough to ask, they're old enough to get an answer. （只要他们问你了，你就可以开始回答了。）

How do adults keep things age-appropriate? （家长如何给出与孩子年龄相符的答案？）

It's about limiting the amount of information you give. You don't want to overburden them with too much and confuse them. Be direct. （家长要注意给出的信息量，不要给孩子过多的信息，适量就行，不要让他们感到迷惑。 一句话，家长应该直接了当。）