This question is like do for yeeeeeeeears.
But in Magic Duel, twilight remarks that age spells are only accessible to the highest level unicorns.
In Uncommon Bonds, Starlight casts a spell to turn herself and Sunburst back into foals.
So the question would be: why did Starlight have access to the ability?
Barring of course plotholes and dumb writing (booh!)
Some theories that came to my mind:
1. Starlight is an exceptionally magically gifted unicorn, so she obvious has easy access to the highest level unicorn spells.
She is the highest level unicorn!
2. Starlight has her way of casting spells through magic scrolls, which actually can contain enchantments beyond what raw unicorn magic can provide. Scrolls can give a significant boost to a unicorn's natural talent.Post too long. Click here to view the full text.