@Derpy’s Age, Her possible motherhood, and other temporal concerns.
Usually I compare the various ponies to a Pathfinder RPG Race of similar… Erm… “Theme” I guess. So, I’ll also assume that their aging works similarly as well.
Now, as Pegasi are essentially attuned with air, weather and stuff, I assume an appropriate comparison would be the Sylph; people with the blood of air elemental running through them. They grow to full adulthood at about 60 years, and reach middle-age at 150.
Unicorns are in some sense comparable to Elves. Which reach full Adulthood at 110 years. And don’t reach middle-age until 175.
Now, if you remember that the Mane six are described as “Young adults” and assume this means that young adulthood could be anywhere bellow these figures, just that they’re kinda immature for their species. One could Assume That Derpy had a chance of it working, but that may be a stretch for some of you.
However, it may be unfair to Compare Sparkler or Dinky to Full Elves, considering that if they were indeed her children, that would make them closer to Half-Unicorns. And thus Half-Elves might be a better comparison.
And since Half-Elves only take 20 years to reach adulthood, and 62 years to reach middle age; one could make the mileage that Sparkler and Dinky could be her children.
Now… These comparisons may not be right for the show. But they satisfy me so. Eh. I’ll keep running with them, see where it gets me.
And it does lead to a little bit of a way to angle it the various ponies’ maturity levels. Especially if one assumes that they’re all around the same chronological age. Which I’m going to arbitrarily place at 18.
-Twilight (Powerful Unicorn=Elf [Full maturity: 110 years]): Although quite well educated, she is known to stress out to incredible degrees when placed under the pressure of… Well… Her studies, time management, etc. So while definitely intelligent, she’s not as mature as she tries to make herself seem. And compared to the others, her freak outs are pretty much legendary. This comparison makes sense.
-Dash (Pegasus=Slyph [Full Maturity: 60 Years]): One does notice that Dash is placed firmly in the grounds of adolescent behavior. She’s loud, brash, arrogant, a little excitable, a need for attention, overconfident at some times and not confident at others. Yep, makes sense here.
Applejack (Earth Pony=Human [Full Maturity: 15 Years]): Well… She does seem to be the most mature and “Wise” of the group. Makes sense.
-Rarity (Low Level Unicorn=Half-Elf [Full maturity: 20 years]): Well, she is almost as mature as Applejack, however she has quite dramatic moments of utter… Well… Just remember her whining? or her drama queen nature? The first was something of an act she put on remarkably easily, and the second is within the realm of a young adult doing. Makes sense to me.
-Fluttershy (Pegasus=Slyph, [Full Maturity: 60 years]): She’s a scaredy cat… Calling her immature for that seems natural, if a bit mean. And considering her freak out reaction at the gala, its probably appropriate. Still less of a freak out than Twilight though.
Pinkie is an Anomaly… So I’m probably going to go with some Logic bending here and compare her to Gnomes, which are the eccentric race among the fantasy races, often thought of as crazy, and known for a penchant with odd abilities; and considered to be related to the fey. (40 years for full maturity).
If one ignores her crazy antics and focus on emotional maturity, she’s only had one major freakout we all remember, but it was exceptionally minor compared to the others. In fact, compared to the others her freak out looked tame in comparison. Closer to being regularly depressed (Eccentricity of talking to herself aside) And she took it better than Dash or Fluttershy took their freakouts. And while still not as mature as Applejack or Rarity, she definitely has more emotional resilience than the rest of the group… And considering that with Pinkie, being “Mature” seems to entail being able to take anything in her stride without loosing that “Childlike innocence”, I’d say this interpretation works well.