Considering how the main cast are depicted, i would actually think C most likely.
It's not like time and age manipulation are beyond magic.
Yes, the target audience is young, but that doesn ot mean the cast has to be. One of the reasons MLP:FiM was such a success was because it did not look down on its audience.
If the cast had to be the age of the target audience, i would also expect the casts activities be more in line with their audience, which is not the case.
In Equestria Girls, all the Mane Six characters are age 17-18 because they are seniors in high school. This is backed up by Sunset's comments on the number of fall formals she's attended, driver's licenses, etc. (There are some strange issues with the mirror universe which suggest it's not precisely IRL Earth, and the legality of a lot of stuff is more down to a kid's understanding, but in general the behavior and age matches up).
Sunset Shimmer is the big exception, of course, and a show that we can't necessarily count on Equestria!Twilight's age as matching up with her apparent age in the mirror universe - it seems the mirror made Sunset a teen, perhaps because 99.99% of the people depicted on the 'mirror' side looked like teens. But it seems very likely that we are meant to consider the mirrorverse Equestria Girls as Equestria equivalents in age.
Although - in 'Magic Duel', Twilight genuinely thinks that Trixie was able to change the ages of Snips and Snails, commenting that "An age spell? But... how could you do an age spell? That's only for the highest level unicorns!" So it's entirely possible that given her prodigal status and experience over Twilight, Sunset Shimmer was able to cast an age spell on herself and keep herself young. As you point out, time and age manipulation spells seem to be a thing. The ramifications of that are certainly interesting.
Given Equestria has what appears to be a pre-industrial economy, the idea that people tend to graduate school at 14 normally and go into business like Rarity or Applejack or take an apprenticeship like Pinkie is actually pretty normal. It's also notable that half of the Mane Six have what seems to amount to civil service jobs, (librarian, weather pony, and wildlife management officer), but that kind of makes sense when you consider that Equestria has had both agriculture and the same government for 1000 years of relatively stable rule.
It's also easy to assume pony lifespans equate to humans both in length and progression, and I think it's a fair assumption given the nature of the show, but it's worth noting that not all animals age in the same
way - the range of health we'd probably consider 'middle aged' or 'elderly' can be either very extended or very brief in some species in contrast to maturity.