You can easily improve by doing speedwork like intervals. The big thing to keep in mind with increasing age is recovery takes longer and you usually need more than 24 hours rest to take on another hard day. You can build muscle and strength but it takes longer and the results aren't as fast.

To improve on century rides, try one long ride each week. Gradually increase the length until you are comfortable about finishing a century in good shape and feeling fine. Then do a couple speed days. Intervals are good at least for one but do "long ones" - minimum of two minutes. For the other day I like doing unstructured ones where I just go hard for a while (whatever feels good at the time), back off, and keep doing it until that's enough. Doing this with friends is fun.