Hello. It all depends on how you are setting up your table. A table with an open top won't keep humidity high enough for little ones. There are many factors to proper hatchling care. Please refer to the species specific section for more information. You can also visit http://www.hermannihaven.com/feeding-tips
By 'not good' I am not sure what you are referring to. If you would like to post a visual of what you intend on doing, we could help you set it up properly but for babies humidity and proper heat is key. Humidity with no heat can cause RIs. Proper UVB is essential as well as diet.
It's more to do with size than age. I'd have said that once it gets to 4-5" (10-12 cm) a table will be fine for Testudo.
The biggest problem is space. Even the smaller species like Hermann's need lots of space. If you have a baby then it will last in a large vivarium for a couple of years and then you need to step up the floor area available - aiming for a minimum of 4'x8' (1.5 x2.2 metres)
Shop bought Tortoise Tables aren't suitable for babies as they have no cover and aren't big enough for older torts. An old bookcase lined with pond liner makes a great tortoise table.
I was told a bit ago, in reference to another tesudo species(greek), around 100g a hatchling can move to a more adult enclosure. Then humid hides, etc, until closer to full grown.
Here's the thing. For babies, you need space but not that much. It's very difficult to maintain heat and humidity for a HUGE space. For beginners, it's best to get proper heat, humidity and UVB in an adequate amount of space so a table is good to start out with. If you make a 4x8 right now, you will have a tough time keeping the heat right throughout your whole floor space. It's also very difficult to do a closed 4x8 enclosure as well.
As mentioned previously, you can upgrade to a bigger enclosure once the baby gets larger 4-5". So tortoise tables are fine so as long as you convert it to a closed environment in the early stages of the hatchling's life.