Actually TL dose means Teens Love, but it's not teenager love. TL originally is a kind of Japanese manga which is target to female readers and usually have a lot of smut content.
TL is similar to Otome R18. MC and ML can be adults , even in their 30s or 40s. But I think in the original TL novel of this manga, both the king and Saint is teenager. The king is only about 3 years older than MC, though he looks like in early or even late 20s （/TДT)/
Last week, me: NO! U murder! don't touch the dear friend! (╯°Д °)╯╧╧
Now, also me: YES daddy ! Please kill that gigolo!ლ(´ڡ`ლ)