All through the nutrients in the ground: Nitrogen, Phosphorus, & Potassium. It's not necessarily how old the plant is, it's the genetics passed from the parent.

The process of photosynthesis is to convert light into energy so that the plant can use it for foliage and/or root growth. So Yes and No, chemical reactions occurs after the plant store energy for use. But the chemical process is done through photosynthesis.

A young seedling with a few set of leaves is not going to photosynthesize as well as a mature tree/bush with tons of branching and foliage. Best I can describe it.

Think about when you were a kid. Did you eat as much as you do now? As a grown adult your body needs more nutrients to maintain health. As you grow the more energy you need. Same as a plant. The bigger, the more foliage it needs to photosynthesize to maintain its health. Hence, chemical reaction process through photosynthesis.