What causes hunger between meals? Is it a sped up metabolism?

These last few years I've been hungry a lot between meals. Usually I'm fine waiting until lunch for my first meal but I usually get hungry before dinner time after eating lunch and this summer onwards I sometimes also get hungry before lunch and have to eat breakfast. My mom says that as you get older, your body's ability to tolerate delays in food intake become weaker and you need regular food to maintain your comfort level, but a young person can go days without food and not feel hungry.

What causes hunger between meals? Is it a sped up metabolism?
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