For spoken languages, it doesn't seem so. Barring outliers like developmental disabilities, every child exposed to a language will learn it. If they couldn't, then the language would die out! There are some things that can take longer to acquire than others—for example, Spanish-speakers will take longer to learn the trilled R than the other sounds, because it requires some precise motor control—but barring some sort of disability, they'll still do fine. (I believe clicks are actually acquired more quickly than trills, but I don't have a source for that. Anecdotally, a lot of English-speakers can do a click and can't do a trilled R.)
For writing systems, there are definite differences—Korean-speakers don't turn knowing their writing system into a competitive "sport", the way English-speakers do with spelling bees!—but interestingly, this doesn't seem to have a major impact on literacy rates. More than anything else, literacy rates depend on people having the opportunity and the motivation to learn to read, and making it easier for kids to go to school matters far more than simplifying the spelling.