When should I start offering the kittens solid food?

I think they are about 4-5 weeks old. They have been observed grooming themselves and going to the bathroom on their own, I was told. Mom is very emaciated though, she probably isn't more than 7-9 months. Just got them today. The mom ate two 3oz cans of kitten food and about 1/2 a bowl of grain free dry food. But she hadn't eaten since early this morning due to long travel distance. The kittens did come out of the crate when mom was eating the canned food but none of them tried to eat it. They had nursed quite a bit so probably not hungry.

Ok after looking at a kitten progression chart I don't think they are more than 4 weeks. 

When they start to crawl into the dish, they are old enough to eat solid food.  Let them and mama decide.  They have millions of years of instinct behind them.

Were these rescue cats?

Basically yes they are rescues. We took them in to save them from going to the shelter. They were my husbands coworkers cat, a female they never spayed and left to live outside. She is really young and emaciated. They said she just showed up with kittens and no one knows how old they are. I don't think they are more than 4 weeks old. I have another thread on them in this forum.

If they have teeth, they're technically old enough. And usually outdoor mama cats don't start bringing their babies along to the feeding area until she's trying to teach them to eat solids. I agree that if they're with their mother, it will be a natural process. . .at some point she'll show them how to eat from her dish. You can help them along by rubbing a little taste in their mouths, or on their paws if they're old enough to groom themselves, but it's not necessary. Although I'm sure the poor mama cat could use the break!

If they are crated with mom while she eats canned, they will get curious on their own around 4 weeks and start poking around after seeing her eating daily.

Soon enough they will start eating it too.