This famous quote in a classic Dr Seuss story can be applied in all of life.
One example is slavery. Every 26 seconds a child is sold into a lifetime of slavery. Most of these children give up hope of ever getting out of there. They are treated as property, not as people. But they are people, nonetheless.
Another example is abortion. Every year, over 80,000 little children are taken from their only means of life, and left for dead. Most people call these little people 'fetuses'. I cannot understand why because 'A person is a person, no matter how small'. And unborn children is about as small as you can get.
So no matter if you are an unborn child, kid, teenager, young adult, adult, parent, aunty, uncle, grandparent or whatever, you are still a person, no matter how big of small you are.