Actually, here's a logical explanation to prove that Derpy is NOT in her 18's:

If the EQG world would have had the ponies turned into younger human counterparts, then how come Twilight Sparkle ALSO becomes a teen if she should be an adult? It's because it turned her through age. It did NOT change her appearance, nor her age. While Derpy, on the other hand, is seen at the age of somewhat 16-17. Now, if Twilight's human counterpart is also a teen, then shouldn't it also be the opposite for Derpy's human counterpart? It would be that the pony age is the same as the EQG age. Now, please tell me you understood nothing or simply call it non-sense.