Showing signs is a different thing compared to getting a diagnosis, so it's tricky. Someone with a personality disorder has to be 18 or more to get a diagnosis but of course the issues does not happen as they turn 18, they can probably show from a very young age in some cases.

Then when it comes to depression and bipolar, I think they happen more for adults and older teens, not saying there are not children and teens clearly depressed because there are. But all of them don't show a classic pattern of depression or slower mood swings. Kids have more energy and I think it is why their depression or bipolar can have more traits of hostility, irritability, agitation, restlessness, loss of control etc.

Anxiety I think can happen at any age basically. Same with depression.

It's really just schizophrenia that seems to have a sort of timeframe for onset, and to a lesser degree bipolar. OCD seems to somehow fit this too.

It's an interesting question. For me, I think I was born with AS and ADD, developed panic at age of 6, transient tic disorder at age 13, OCD at age 14 and depression at age 32.