Age has nothing to do with it. I am almost 59 and I am one of the youngest riders in the group I ride with.
As far as the numbers on the shifters. You don't need to know the number. It just begins to feel intuitive... I don't ever think, gee, I am in the 50-15 gear. I bet you know when you need to go to a lower or higher gear! So, maybe it's that you are getting confused about which way to push the shift levers. That just is practice and memory. Don't focus on what gear you're in, just how it feels. Start riding in the small ring (left side) and play around with the gears in the rear (right side). See how each one feels. Then do the same thing in the big ring. Do this on flat ground, so you can really feel the difference. When you get that, start making switches between the big and small ring on the front. Experiment with different combinations. You don't have to know what the numbers are, just how it feels. Eventually, it clicks.
There are people who give riding lessons; maybe you can find someone who is willing to help you.