1. There’s nothing to say that each season represents a full year in Ponyville. We haven’t seen any(?) birthdays or holidays repeat. It could easily be only a year or two that we’ve been following these characters.
    
2. It is completely possible that ponies have a different social outlook on maturity than we do. Without getting into the reasoning and politics, I’d argue the ponies are a lot smarter about it than we are.
    
3. It’s been stated by show staff that Rainbow Rocks takes place only a few months after the first film. It can also be assumed without too much of a stretch that time in pony-world is different than time in human-world. It seems like a lot longer time passes for ponies than people, which would explain things like the CMC going to the same school as the Mane 6. Incidentally, we also don’t know what, if any, the grade system is like at Canterlot High. Is it a standard four-year high school like a lot of US schools do, or is the age scale much broader? We don’t really know.

@Molly the Moopy
It wouldn’t be pedo, she’s an adult in the show and it’s implied that she’s a senior in EQG, making her around 18.