Are financial institutions less likely to lend me money because of my age
Yes. But they are especially unlikely to loan you money because you have little income.
or because they know I avoid interest by paying things off aggressively?
This won't affect them. But you might ask yourself how much credit history you have. Credit history can include all of loans, credit cards, rent, utilities, etc. You mention three loans. But you don't mention rent or utilities. You may simply not have much credit history, even if what you do have is good.
But again, the biggest thing that they will look at is your income history. If you have a small income, then it doesn't matter what your payment history is. They don't want to loan money to people who need money. They want to loan money to people who don't need to borrow but are instead bringing a future purchase into the present.
The ideal recipient is someone who has a high income and spends it all every month. Such a person is likely to borrow heavily but be able to keep up the payments.
Obsessing about your ability to borrow is probably the wrong approach. Instead focus on how you can meet your goals without borrowing. Eventually your ability to pay will catch up. Then they'll offer you money. Of course, you might not need it then.
Note that when I say little income, I'm talking about their perspective. You may be fully on track and making decent money or even very good money for your age. But they're looking for people who are mature in their careers and regularly bringing home large sums but who spend it faster than they can get it.