When young, I am sweet in the sun. When middle-aged, I can help to make you healthy. When old, I am valued more than ever What am I?

“Honey”

While honey is sweet and can have health benefits, it does not go through a 'middle age' or 'old age' stage like grapes do. Honey is a product rather than a natural life cycle.

“Banana”

Bananas are sweet when ripe, but they do not have a significant health-related middle age or old age stage. They do not become more valuable as they age in the same way grapes can become wine.

“Apple”

Apples can also be sweet and healthy, but they do not age in the same way that grapes do. The riddle's emphasis on value increasing with age aligns more with wine made from aged grapes.