Tense in the novel

Hi there,

I'm busy working on a novel that is divided into two parts, two important years in the life of my character.

In the first part the protagonist is a child and the narrative is past tense, first person. In Part 2, my protagonist is an adult and the narrative is present tense, first person.

Has anyone out there actually read anything successful that works like this? (in two different tenses).

It wasn't something I planned, it just came out that way, and everytime I sit down to write in the older characters voice, it comes out in the present tense. It does seem to flow, and I am considering having some flashbacks, which will go back into the past tense.

I'm worried though, all those creative writing gurus spouting stick to one tense. What are your esteemed opinions?