Nie no Akazukin

When a lady gained magical power and became a witch, she stopped aging. In a forest close to the village where protagonist Lilia (name changeable) lived was an old witch who had aged a lot before becoming one. That witch wanted a remedy that would restore her youth. One of the ingredients was a girl whose age was around that which the user wished to return to. The remedy gradually lost its effectiveness as time went by, taking around 10 years for its user to go back to normal. In exchange for the witch to cast a spell that protected the protagonist's village from getting attacked by the forest's beasts, its residents offered the witch a suitably-aged girl every 10 years. However, the residents didn't want to escort the sacrificial girl all the way to the witch's house. Also, as the witch had poor legs, she didn't want to go to the village to pick up the sacrifice either. So the residents lied to their sacrifice. "There is an old lady who lives alone in the forest. She's sick, so go check on her." Because the witch had poor eyesight, the residents garbed the sacrifice in eye-catching clothes and made her go to the witch's house alone. To hide the sacrifice's true purpose further, the residents never called her a "sacrifice". They named her after the clothing they garbed her in. "Little Red Riding Hood". Due to her stepmother's scheming, the protagonist was unfortunately selected as the "Little Red Riding Hood" of that year. However, it wasn't the old witch that had awaited her. It was two wolves whom had eaten the witch and gained her magical power... [Translation of DLSite Japan's story summary.]