I think you are
I am something people love or hate.
People tend to love or hate their own age depending on how old they are.
I change peoples appearances and thoughts.
People's appearances and thoughts change as they age.
If a person takes care of them self I will go up even higher.
People live longer if they take care of themselves.
To some people I will fool them.To others I am a mystery.
Some people do not look their age, some people do not know how old they are.
Some people might want to try and hide me but I will show.
Some people try to hide their age using cosmetic methods but eventually it will show.
No matter how hard people try I will Never go down.
Age only increases with time.