During your time with Maneater, you'll grow your shark from a small pup all the way to a mega that dominates the waters. The Elder level (also refered to as Elder size or age) is the last of the intermediary steps in that journey.
To help you better map out the sizes you must go through, here's everything you need to know about when and how to reach Elder level in Maneater.
Maneater Elder Level
Maneater's Elder level becomes available upon reaching level 20. As opposed to other ages which require you to pay a visit to your Grotto in order to grow, Elder asks that you take the fight to your nemesis.
As soon as you reach level 20, you'll notice in the lower right corner of your screen text telling you to "Fight Scaly Pete". You'll then have to progress through the remaining story missions until the battle with Pete becomes available.
Upon completing the fight, which can be seen as Maneater's first proper boss battle, you'll reach Elder level which means you'll more easily dominate the wildlife in Port Clovis' waters.
Getting to level 20 is best done by following the story objectives, eating stuff and collecting nutrient caches. If you really need a boost, pick a fight with some hunters and kill as many as you can.
Reaching Elder age in Maneater also means that you'll be able to break open new grates in the environment, opening the way to previously inaccessible collectibles.