In order to protect my age of credit, do I need to not pay off my mortgage?
No, pay off your mortgage. It would be stupid to pay interest as an effort to keep your credit score higher.
Your score might drop a whole 10 points per https://www.fool.com/the-ascent/personal-finance/articles/what-happens-to-your-credit-score-if-you-pay-off-your-mortgage/
my oldest cards were closed from lack of utilization
I set up my oldest credit card on autopay for my Internet bill to avoid this scenario.
This isn't a superfluous expense at trying to trick the almighty credit bureau; it's a necessary planned expense so it's okay to just autopay it.
Because if I were to pay off all my cards, and never borrow again, everything would age off in a few years and my excellent credit would disappear.
Correct, don't let your credit history go stale for 10 years.
It's easier to keep a credit history active by maintaining 2-3 credit cards for an extended period of time than it is to start from scratch; albeit not impossible.
As an 18-year old I got my first credit card with a $300 limit; presumably I had zero credit.
As a 23-year old I got my first car loan on a score of about 720; paid off after just 4 years.
As a 29-year old I got my first house with a credit scored of about 810.
We all started somewhere so for you it's just a matter of not starting completely from scratch because I would bet that a $300 credit limit does not sound enticing at your age.
Lastly, a loan is not impossible to acquire if you have zero credit. A creditor will assess your occupation and assets to determine if they can trust you. The interest rate might not be stellar and collateral could be required but if you need the loan then you'll take it, no?