Arch-Demons don't really need their age when they have seven hearts, they barely age over 3,000 years of the time anyway. They have no sense of their age and don't ever keep growing after their decided age to stop aging.

Once per short rest you can activate your Demon mark to gain immunity to Necrotic damage and resistance to slashing and piercing weapons, during this you are vulnerable to Radiant damage.

When you make an attack you may add 1d6 Necrotic damage to the attack (at 5th level 2d6, 10th level 3d6, 15th level 4d8)

The transformation lasts how many minutes equal to your charisma Modifier.

Your eyes turn a dark purple, with your pupils a bright white, dark shadows cover half of your body, and shadowy wings sprout from your back

You gain 60ft of flying speed when active and take no fall damage

Your eyes turn a deep shade of purple in the dark, you gain 60ft of dark vision, and can see in darkness as if it was a dim light, and dim light as if it was shining daylight

No matter how hard they try to hide, arch-demons always seem to scare off people when they show up. However, some are easier to fool than others and maybe easier to blend-in around

You are proficient in Intimidation and Deception

+2 to strength and dexterity, and +1 to charisma