When an old ruin is unearthed under Ponyville, it will take the boldest and most adventurous of ponies to dare sound its depths. Daring Do knows she's such a pony.
So what if she's eight and a half, and Dad says she can't go? That's a big age to reach, and he doesn't have to find out. So what if millennia-old dangers and secrets lurk in the darkness? That's the point of adventures.
She's got a saddlebag full of useful tools, countless stories telling her what ought to happen, and her Mom's old helmet. You don't need much more for your first adventure than that.
