What clues indicate Pip is narrating in a more mature phase of his life and what is the effect of this kind of narration?This answer is in chapters 1-9 of Great Expectations
Since Great Expectations by Charles Dickens is a bildungsroman, or novel of maturation, there is a growth in Pip's perspectives as the narrative proceeds. That having been noted, there is, however, a subtle tone of maturity expressed in the earlier chapters that goes beyond that of the small boy involved in the narrative.
This duality of the narration is evident in the word choice and narrative comments that Pip makes about the other characters with whom he becomes involved. For instance, in describing Mrs. Gargery, Pip narrates,
My sister, Mrs. Joe Gargery, was more than twenty years older than I, and had established a great reputation with herself and the neighbors because she had brought me up "by hand."
This observation that his sister is smug about herself is clearly from a person of some maturity. Likewise, Pip's detections of Uncle Pumblechook's pretensions are also those of a more mature narrator. In Chapter VIII, Pip describes the corn chandler's premises,
Mr. Pumblechook's premises were...of a peppercorny and farinaceous nature, as the premises of a corn-chandler and seedsman should be.
Pip further observes keenly that Pumblechook conducts his business by watching the shopkeeper across the street, who, in turn, watches the saddle-maker, who then watches the baker, and so on. Only the watchmaker, intent on what is before him, seems to be the only person on High Street "whose trade engaged his attention." This passage also demonstrates a mature, wry humor untypical of such a young boy as Pip is at this point.
One very apparent example of an older Pip as narrator appears in Chapter IX in which Uncle Pumblechook and Mrs. Joe are almost ravenous in their curiosity about Pip's visit to the home of the wealthy eccentric Miss Havisham:
The worst of it was that bullying old Pucmblechook, preyed upon by a devouring curiosity to be informed of all I had seen and heard, came gaping over in his chaise-cart at tea-time, to have the details divulged to him.
And, previous to this passage, Pip narrates,
I felt convinced that if I described Miss Havisham's as my eyes had seen it, I should not be understood. Not only that, but I felt convinced that Miss Havisham too would not be understood; and although she was perfectly incomprehensible to me, I entertained an impression that there would be something coarse and treacherous in my dragging her as she really was....
As a more mature narrator who relates his story in retrospect, the reader benefits--as does the reader of other novels, such as To Kill a Mockingbird in which the adult Scout narrates--from the insights of one who reflects upon events with a measuring bar of experience. Such narration makes for a novel that can be enjoyed by younger and older readers alike.