white spots on my turtles' shells: good or bad?

by Nafisa

my turtles(red eared sliders) have recently developed white spots/marks on their shells especially by the borders. so far two out of four turtles have shown this sign.the marks dont stink, or come off. they are also not squishy or soft. they are just as tough as the rest of the shell. Is this the signs of them growing up, (they are about 5.5cm big now) or is it something bad? PLEASE ANSWER!!