Is there an age requirement for demand deposit accounts per regulation for teenagers to open an account without an adult signature? If not, are there any other stipulatons for this type of account?

The age requirments will be governed by your state law.

We've always gone with the age of majority. If they can legally enter into contracts, they can have a checking account without an adult's signature.

I agree - we go with 18 years old in order to have an individual deposit account. If a minor overdraws an account, you wouldn't have recourse for collecting the funds.