Older one are a lot more easier I care for. Specially when your new to the tortoise world. That's just my thought. I would go into details but I'm on my phone so ill post more about it a little later.
With a Russian or Greek the adult/juvenile care is similar to a hatchling but not exactly the same. My hatchlings get a higher humidity in a glass indoor closed chamber and their food gets chopped. They are removed, soaked and sunbathed everyday. All of my adults live outside yearround and are encouraged to forage for themselves. So hatchlings are more delicate and exacting with a greater chance of failure.
When I started, I went for yearlings because the hard work is basically done. Hatchlings require more attention and the first year is toughest. The daily soaks, feeding more often, cleaning the enclosure more often, etc. So getting an older one is easier, especially if these are a child's pet.
BUT, now that I have hatchlings, I can definitely see the other side too. Yes they require a lot of work, but darn they are cute and you get to watch them grow and develop personalities. You get to control their environment and food so you know they get raised properly.