Does lethality scale past level 18 in URF?

For reference, the Lethality formula:

Flat Armor Penetration = Lethality × (0.6 + 0.4 × Target's Level ÷ 18)

Does it scale higher against targets higher than level 18, or is it hard-capped to the amount of Lethality the sources have?

(15 Lethality would scale up to 19 armor penetration against a level 30 target, if it does work that way.)