Dickens does not reveal Oliver's exact age at the end of the novel, but he does have the kindly, eccentric Mr. Grimwig, a reliable character, state near the end that:
the devil’s in it if this Oliver is not twelve years old at least
Context clues would confirm that Grimwig is not far off, though it is possible that because of his malnourishment, Oliver may be a year or so older than he looks. We know he cannot be much older than twelve because he is referred to as a boy at the end. He is not old enough to live independently, and we learn that Mr. Brownlow adopts him. Oliver's youth is attested to as well when the narrator states that Mr. Brownlow filled the:
mind of his adopted child with stores of knowledge
Although Oliver is relatively young at the end of the novel, the story is nevertheless a bildungsroman. A bildungsroman is a novel of a young person journeying from youth to maturity and being formed into the person they will become. Oliver's various adventures up to the age of twelve have been largely horrible, but they have also given him the point of view of the underdog. He develops a compassionate heart as a result of his sufferings. He also is able to look at his youthful terrors from a vantage point of greater security. For example, he is able to go past his old workhouse without his former fear.