I like the theory that the symbols change or become more complex as they age. I'd like to see someone write something for Baby clones that grow up and find their own identity as they age! Also a bit off topic, but this reminds me of the baby care bears with their very baby-ish tummy symbols.
I seem to remember a lot of ponies having innate magic, like SS Twilight putting out candles with her ear wiggling, maybe the symbol just defines what their magic effects?
Here's some overthinking fancontent fodder:
Spoiler
The clone babies all have their symbols because they are, of course, clones. Whatever gave their mothers their symbols found its way into each of the babies. I'd imagine that once they are no longer Babies, to avoid confusion, they all adopt their own names. Perhaps the marks are left largely unchanged, but adapts to fit that pony's magic (which may be affected by environment/will/training). Although Gusty can control the wind, Baby Gusty now has the ability to advance aging somewhat (leaves, fall, you know?).
Ponies with no symbol can earn one by learning magic, or maybe Ember was a clone baby gone wrong? Hence the triplets and the one with a symbol. Ember has a lot of potential.
Baby ponies with baby-related stuff might see their marks adapt to a more all-ages mark (a pacifier? Well, now it's a Lollipop! A rattle? Maracas!) or they could just be stuck with a caretaker role. I like the former because I'm trying to distance away from G4's "You are the government assigned babysitter" thing.