I have seen things like this in advertising from foreign and American companies. I think that sometimes this is done to draw attention to the item and sometimes it may be used as a slang phrase. It isn’t just you—I have seen things like this that I thought were odd, too.

Thank you everyone for your good contributions.

I definitely jumped the gun and that's on me, but I'm glad it generated a nice discussion.

You are right that this is an example of grammatical flexibility. It’s not wrong, and there’s nothing to fix. It would actually sound worse if it were “Be Younger”. If the brand had “Be Younger” instead of “Be More Young”, I would have thought that they wrote their copy using Google Translate, because it’s technically correct but doesn’t fit the context and lacks grammatical nuance.

I guess they assume that the advantage of having their brand name seared into people's minds outweighs the impact of any letter from linguists.

It reminds me of the many discussion about the Toyota Aygo car advert a few years ago. The one where they say "Aygo by Toyota" at the end. It is the Aygo car model by the Toyota company, so it works. Are people constantly thinking "I go buy Toyota" when they hear it.

Marketing departments can be highly manipulative. It wouldn't surprise me. You may be right when you say it makes you think more about it; we're all discussing the brand now, so I guess it's worked! Or, it may be more subtle and just slowly gets into people's heads.

Michael:

From a descriptivist perspective, a native speaker by default does not make any mistakes. An individual variation would be considered an idiolect (which is essentially the dialect unique to a specific person). However, I don’t think this applies to orthography (spelling). The reason is that orthography is not inherent to language. It’s a representation of language. I can write English using the Arabic script and it’d still be English, not Arabic. Orthography is not natural the way language acquisition is natural. Illiteracy is a prime example of that. You can spend your whole life speaking a language without knowing how to read or write it.

Elena:

It’s difficult to say where the line is. There are a lot of nuances in language usage. I use different styles when I write on italki, when I write on social media, when I text my friends, when I write for creative purposes, etc. As someone with training as a writer, this is something I am very aware of and use to portray a certain image when I’m writing. The style that a business’s marketing copy is written in is part of its brand image: whether it wants to sound friendly or professional, for example, or whether it wants to sound “well-established” or contemporary.

For a learner, the question is less about where the line is and more about what is useful to learn. This depends on the learner’s level and their purpose for learning the language. A learner learning English to go to university needs to have a more formal knowledge of English. One learning English to write advertising/marketing copy might need to have a more creative style.

When you come across something that sounds off/wrong, I think it’s better to ask why it was written that way and how it fits into language usage, rather than asking how it’s wrong and questioning if your knowledge of grammar has been wrong all this time. The grammar learners learn is not wrong, but it’s a simplified version that is used to make learning the language easier.