Also, growth is a skill, and comes in two varieties, bell curve and flat.
A bell curve growth is one where a character grows faster the longer they live. The flip side to this for a story is that, often, a character doesn't grow too quickly to start with. So for example, farmer boy Tom needs to learn how to swing a sword. He spends several IRL months learning and perfecting sword swinging. In a magical setting, this probably isn't too phenomenal in terms of power growth in comparison to, say, becoming a master wizard and blowing up towns, or a super ninja who can turn totes invisible while wearing orange.
However, once Tom has all the basics down, he quickly learns other, demonstrable stronger techniques, faster. This usually goes under the premise that one learns how to learn faster by learning the basics of learning. As well, Tom can now likely find his way to powerful artifacts to help amplify his basic power, like a fire sword, or a lightning sword, or a sword that makes dead grandparents appear to their enemies to tell them how bad they're being today. Obviously Tom's power curve has suddenly sped up, this might all happen in the course of a couple months instead of the several it took to make Tom a competent fighter. Welcome to the Bell Curve: The stronger you are, the faster you grow, because it enables you to grow faster.
Naturally Bell Curves are better in stories with a definite ending, like rescuing the princess from a tall tower that is being guarded by a dragon. In literature and film you most often see Bell Curves in Coming of Age type stories.
A flat growth rate is exactly how it sounds and is easily the most manageable: Every "level" a character has they gain 10 points in X skill or stat. It always grows at this rate, there is nothing dynamic to a flat growth rate. However, that being said, it's very easy to manage and apply said growth rate to the antagonists of the protagonist as well. If the protagonist gains 10 "strength", then maybe the antagonist gains 10 "magic". Bell Curve can also do this but it can be harder to logically map it in comparison to flat.
Just remember: Antagonists should grow, too. If the heroes are suddenly capable of punching out Cthulhu, then the Antagonists should thusly now be capable of punching out their God. The only thing that should remain consistent at its own power level is the setting itself: If you're in modern day New York as a super hero, and you keep growing in power, the amount of collateral damage you could incidentally do to New York would grow with each step you grow. The only difference is that the villains just don't care about such things and will happily murder four million people if it means taking over the rest of New York. Growth in a super hero sense, when handled properly, merely means upping the ante. Where it gets silly is stuff like Superman sneezing destroying a dozen solar systems. (Yes, this happened.) Aside from that, go wild.