Maine probably had more settlement earlier than any area west of the Appalachians, even if not so much as the other Atlantic seaboard colonies.
The simplest one here is just to have Maine be its own chartered colony pre-revolution.
Possibly it never gets absorbed into Mass. Or, an early dissident group from Mass sets up its own colony.
Or there is simply a decision in the 18th century to spin Maine back off from Mass.
Perhaps as late as the early 1770s
Post Utrecht or post-Dominion of New England might be the best times to do this. A lot depends on if there was ever local demand.
But, however it occurs, as long as Maine matches the sentiments of all the other colonies with regard to independence, 14 colonies is plausible from the start. The “free vs slave state” balance, when it becomes relevant in the nineteenth century, starts with one more free state. This might diminish southern influence in the Senate, or cause division of territories in the southwest into an additional state.
The knock on what-if here is what if there was an additional southern state carved out of the transappalachian south? How would it be shaped? What would its demographics be? And what consequences would this have for later sectional conflict?