Free Use Universe/Age Chart
This describes the general breakdown of what sexual acts are considered safe for what ages of children.
Once they have started puberty (in this universe, 5-7) they are free to use, but they may still adjusting to their role. By the time they are 8, they are fully comfortable both being used and using others, and you can fuck them however you wish. (Of course you can't harm them, but that's true for all sex acts).
You can still have sex with kids who are younger, but you have to get their consent. However in practice this includes many situations that some might deem dubious, where they aren't exactly saying yes, just not saying definitely no.
Parents have full authority to consent for their not-yet free-to-use children, so getting their approval is also acceptable. As a result, parents can require their children to participate in any sex act, including both sibling/sibling and parent/child incest. Generally they follow the guidelines below. (Once a child has started puberty, they are free to use by everyone, including their parents, so parental consent is no longer relevant).
Everybody is able to have sex with free use individuals, even if they are not old enough themselves. The chart below is for if you wish to have sex with them, not if they want to have sex with you.
Some actions (such as ejaculating on or taking pictures of somebody) never requires their consent, and can be done wherever you wish, regardless of age.
