Can someone explain the differences? I know what Yaoi means and I know what shounen ai mea...

Shounen Ai means no sex at all. It can go as far as being only a teaser (╬ ˘^˘ )
What I noticed is that usually when the two are together is because there are two or more stories in the manga and some of them are Yaoi and some of them are Shounen ai

I think shounen ai has kids or more like the characters are younger or something.

The main difference I see when I'm reading (I read both about the same) is that Yaoi is guaranteed to have sex scenes where as Shounen ai is more about the couple, heavy making out but little sex if any. Hope that helps.

Shounen Ai/BL has no explicit sex scenes. Sex will be implied - the characters kiss on a bed and the next frame is them in bed in the morning talking, naked above the waist, as if they just had sex - or simply missing entirely in the story. Yaoi has explicit sex, explicit language, adult themes and/or situations. Even if it is heavily censored, it still counts as yaoi. Shounen Ai is available for sale to all readers and yaoi is restricted for sale to readers over 18. When a manga is tagged here with both shounen ai and yaoi (since the tags are placed by both readers and by the uploader who is not associated with the original publisher) there are two possible scenarios: 1. The tag was put there before the manga was completed and the yaoi scenes had yet to be read. 2. There are several stories within the manga and one or more would be considered shounen ai. A manga that is published and sold as shounen ai cannot contain yaoi, but a manga that is published and sold as yaoi can contain shounen ai. So, if you're in a bookstore looking for a manga that contains both yaoi and shounen ai, look first in the yaoi section because it can't legally be sold as shounen ai. Many readers assume that BL and yaoi are the same, but one is restricted and the other is not. It's like saying grape juice and wine are the same. They come from the same source and contain most of the same ingredients, but the one crucial ingredient, alcohol, is missing in one and present in the other. Making them very different. In the case of BL and yaoi, the crucial ingredient is explicit sex.