Twilight and Spike are having breakfast. Spike asks to go see a new movie that's come out, but Twilight says that as a baby dragon, he's too young to stay out that late (night viewing only). Spike goes quiet at this, and Twilight quickly notices that Spike is more distressed than she thought he would be. She asks what's wrong.
Spike asks Twilight how she knows he's too young. When Twilight looks confused, he reminds her of his greed growth during his birthday. He then asks, if dragons grow via hoarding and he doesn't hoard, how does Twilight know he's not already meant to be a teenager/adult. Twilight has no answer to this, which depresses Spike.
Wanting to answer his question, as well as satisfy her own curiosity, Twilight calls the other girls to the library the next day (Spike is not present). She tells them about what happened and asks them what they think. They each take sides (In my personal opinion, Fluttershy would support him still being a baby, while Rarity and Applejack take his work ethic into account and see him as a young adult). After some debate and discussion over what exactly makes someone an adult besides their body, Twilight comes up with an idea.
Over the next few days/weeks, and without Spike's knowledge, they'll put Spike through a series of tests for various aspects of adulthood (mental/emotion maturity, skills, ability for other ponies to see him as a kid/teen/adult etc.) and use a scoring system to determine what he is and how they are to treat him from this point forth. They all agree, with some making bets (Rainbow), and get to work devising the tests.
The bulk of the story would be the tests themselves, as determined by the author. It would make sense to include the CMC in at least one of them, though, since being a school-aged dragon is also a possibility.
There are several possible endings depending on what the author likes and how the tests work out. My personal favourite is Spike finding out he's being tested, and confronting the girls over it. He berates them all, saying that he trusted them, before walking off. The girls realise that what they did was childish, and that Spike was actually more mature than the 6 of them combined during the whole ordeal. Because of this, they decide that Spike is a full-fledged adult, and go to tell him this, as well as apologise. Epilogue would be all of them going to see the movie.