Yes, it's because of your age and yes it's because of the layoff from riding.
When I started back 3 months ago, I hadn't rode in over 10 years. I was even a little nervous just mounting up for the first time. About 30 minutes into, it started coming back to me. And even now, I'm not the best at cantering, but I'm coming back into it. It'll just take a little time to get your seat back and remember how it felt.
Good luck! And the instructor idea is a good one. Even just once or twice a month could help you progress better than if you just did it alone.