As my girl got older much of her pink skin turned dark and she looks like she has big freckles on a lot of her body. She is quite spotted now, if she had no fur, she would look much like a Dalmation! She is also light colored so you can see it more.
I think it is because they are maturing but I am sure others will give more input if it is a sign of something. If there is no itching or hair loss, I think it is a normal process of their growth.