It's not "we don't trust you", it's more of "you need to be older and have a basic understanding of the consequences of what you're doing".

People under 18 can have sex perfectly legally as long as both parties consent. The parents may object but there isn't a legal process to punish them.

Under 17 can buy video games, just not ones rated 17+. Because moral crusaders believe that people shouldn't be allowed to make their own decisions for the entertainment they consume and enjoy. I've seen plenty of people who couldn't handle an M-rated game that were over 17.

Under 16 is probably more so to protect kids from child labor. As in, you cannot work a 10 year old in the fucking coal mines.

I really don't know what this last point is supposed to be about. I assume it means transgender youths transitioning young. Though I think almost every single doctor will just prescribe puberty blockers that stop the body from producing large amounts of testosterone or estrogen until later in life when they get prescribed HRT. I don't think any doctor would be willing to perform surgery on a minor that invasive. Also not every trans person is the same some do realize they are trans very early on, others don't find out until later in life. If you aren't talking about trans people when I don't know what "change legal status or body altering surgery" means for the LGBT+ community.