The age of a Demon almost always correlates with their strength, for multiple reasons.

Firstly, time allows for the consumption and conversion of "food" to demonic energy. If a demon got loose in a town and gorged themselves they still would not immediately skyrocket in power. It must be slowly broken down, through an unknown internal process. 

Secondly, the more "food" consumed in one go, the less efficacious. Slow and steady intake of high quality materials produce the best foundation. 

Thirdly, the more power a Demon accrues the more resources they will be able to funnel towards themselves. The highest echelons of Demon society are only interested in this, to the exclusion of all else. Ironically this can limit what they are capable of, and stuck in the Demonic Planes as they are, they  end up plateauing. 

The final, and most mysterious, reason is that there was a "first wave" of Demons that seem to be capable of FAR greater energy conversion and energy manipulation. Why they differ so greatly is a closely guarded secret. Still even this group must obey the requirements for energy consumption and expenditure. But with both time AND conversion advantage, we can assume most Demonic Masters have this origin.

If you seek to contract with a Demon, make sure that they have enough strength to actually make the "favors" owed worth it. A Demon is capable of remembering Favors as if they are a hard currency. Spend yours wisely.

-Forbidden text on Communication and Contracting with Demons