It helps them wear their beaks down and it's calcium. I have a little dish of CA powder in it. I crushed up the cuttlebone into little pieces and put in that dish. Now my tortie chews on it. Before I guess it was too big for him. As long as he gets CA by powder over food, they don't really 'require' a cuttlebone.
Just leave it in the enclosure, when their instinct tells them they are short on calcium they'll eat it all. Mine ignore theirs for months, then in a day it's gone. It's their instinct.
This is Novalee's cuttle bone. I'd throw this away, but as you can see, she's a tiny bit attached to it. Leave one in the enclosure. One day they might get a taste for it.
It's not that they "need" cuttlebone, what they need is the source of calcium carbonate. Most very young tortoises won't eat cuttlebone, because it is too hard for them to bite. But as suggested in the posts above it is a very good idea to have it available to them, even at a young age because they will almost always eventually begin chewing on it. To encourage them to do so sooner I break it up in small pieces and scatter it around their enclosure. I have seen some of my yearlings sniffing and chewing on it often. It also helps to soften it by spraying or soaking it with water.
In the meantime they should also have another source of supplemental calcium carbonate in a powdered form. You can purchase it already in powder form or scrape it from cuttlebone.
I always offer it seperately in a shallow dish rather than sprinkling it on their food directly. This way they can control their own calcium intake.
I always leave cuttlebone in with my water turtles, boxies and my tortoise. Even my little hatchlings have some in there viv's. They all take a nibble at one time or another. I think it really helps with their beaks too.
My Russian hatchling never paid attention to his cuttlebone until I held it in front of him one day and he nibbled it. I have since broken it into several smaller pieces and now he will eat them!
I also offer him a dish of calcium powder- he's kind of an addict
I think it is best to leave it in, in addition to the supplementation. My tort likes to walk over his on the way to other things...and has only ever taken a few bites. I call it his surfboard!