Neptune is the second of the generational planets, spending fourteen years in each sign. Everyone born within that period will have Neptune in the same sign. By itself, this placement is not the most important factor in your chart; however, it symbolizes what was going on in the world at the time you were born, and this has left an imprint on you.
You and members of your generation are each imbued with this particular energy, and in some ways it characterizes everyone born during this time. Each individual will express it in their own way, some more strongly than others. As a group, those born with Neptune in Sagittarius are very idealistic about religion, philosophy, education, and travel. Deception or false illusions regarding these areas is also in the mix; the ultimate purpose for you and your generation is to refine and purify spiritual practices and faith.