In Japan, there really isn't much of a distinction. Yuri is relationships between females. Shoujo-ai is specifically relationships between young girls. (making it a sub-genre) As a result, shoujo-ai tends to be more innocent, and so a long while ago the western fandom sort of misunderstood to think Yuri=mature content and shoujo-ai = innocent romance.
But things we would call shoujo-ai by that standard are often labelled yuri in japan, which leads to confusion, and inconsistent classification, which really just makes the distinction between the terms confusing and pointless.
Ah!! OK, that makes sense. I missed the younger aspect of shoujo-ai. Thanks!
The line between yuri & "romantic" shoujo-ai is so blurry... I really don't get the difference.
At this point the only real difference I can see is "romantic shoujo-ai" <= second base, and yuri = anything goes.