What examples would you cite to show how Huck comes of age (i.e. loses his innocence) in this novel?

There's many examples you can cite for this question. Even
at the beginning of the novel, Huck is exposed to abuse and violence at the hands of his
father, which can destroy the innocence of childhood. But Huck's coming of age is more a
rejection of the hypocrisy of the society around him. He does this in several ways:
challenging the traditional gender roles (his respect for the Wilks girls shows that he
values the intelligence of women), noting the discrepancy in the words of many religious
people (Miss Watson's version of Providence), and most importantly, seeing Jim as a
human being, rather than a lesser slave. This particular example of Huck's coming of age
takes place gradually, as their friendship builds. Yet there is a moment in chapter
23 when Huck realizes that Jim cares for his family just as much as a white man would.
Huck awakes to find Jim crying, and describes his
reaction:

I
went to sleep, and Jim didn't call me when it was my turn. He often done that. When I
waked up just at daybreak he was sitting there with his head down betwixt his knees,
moaning and mourning to himself. I didn't take notice nor let on. I knowed what it was
about. He was thinking about his wife and his children, away up yonder, and he was low
and homesick; because he hadn't ever been away from home before in his life; and I do
believe he cared just as much for his people as white folks does for their'n. It don't
seem natural, but I reckon it's
so.

We can see how Huck and
Jim's relationship has developed over the course of the novel. At first, Huck only knew
Jim from the times he and Tom made fun of Jim's superstition. But now, Huck realizes
just how much Jim cares for him. That in itself shows growth. But we also see a shift
from before, when Huck is shocked by Jim's desire to rescue his children from slavery.
When Huck first hears of Jim's plans to rescue his children, he is shocked. Huck
considers Jim's children property of their slaveowner, and doesn't recognize the deep
connection between Jim and his family.

Yet here, Huck's
maturation is evident. He understands that Jim does indeed have the same emotions and
relationships as all humans, although he remarks that it "doesn't seem natural". This is
an important moment for Huck, and it sets the stage for his decision to "go to hell"
instead of turning Jim in later. That in itself would be a good example of Huck's coming
of age as well. His decision to refuse the "morality" of society, and instead follow his
own conscience is the very definition of maturation.