It’s not hard rules, though. There’s a myriad of publishing styles. Each define different rules and guidelines to when and where numbers are spelled out. Hyphen was dropped from several guides, for example. The and has also been optional for certain publishing houses for a while, but in England it is still mandatory. Academic and literary will differ in how they enforce this guides and exactly what they are. Language is relative, changing and fluid, and this was all different mere 30 years ago. It moves with the expectations of the audience.

Also, it is six seven. Respect the memes guidelines.