...and she's still 15? How?

Apparently, her use of the hourglass makes her age a quicker than normal. As what was mentioned before in the manhwa, it seems as if the hourglass takes her life force and this causes her to look more mature than what her age normally is. Which also became an advantage to her somehow as she looked incredibly prettier and womanly as opposed to Mielle: who fits their age bracket of a young teenager of around 14 or 15 years old

No i guess it is normal,
She grew up fast too in the past of her life