The form only appears when the user is rage or in Rage. The user gets a boost in attack damage +2, and is send into a mind clouded with a thirst for blood.

This give the user the ability to fly, conjuring wings out of their back or their arms become wings. They have a flying speed of 30ft.

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

These eyes are known to be the Demon Clan's trademark trait to figuring out who is stronger based on how dark their eyes can become, a few Demon Blood users be having Purple Eyes to show that they are compatible with Demon Blood (Boost Intimidating Presence to have one more turn)

The way you figured out you've found a Demon is by the bizarre and traumatic markings that be placed on their body. The Demon's marking are based on their background and are more efficient in battle if they have been used with consistency. (If you roll 1d10 you can half or dodge attacks, similar to Uncanny Dodge)

you can use your action to frighten someone with your menacing presence. When you do so, choose one creature that you can see within 30 feet of you. If the creature can see or hear you, it must succeed on a Wisdom saving throw (DC equal to 8 + your Proficiency Bonus + your Charisma modifier) or be Frightened of you until the end of your next turn. On subsequent turns, you can use your action to extend the Duration of this Effect on the Frightened creature until the end of your next turn. This Effect ends if the creature ends its turn out of Line of Sight or more than 60 feet away from you.