You should just enjoy the ride. There are advantages to higher cadences, and you should certainly try to minimise the strain on your knees by selecting a suitably low gear, but if you have just started riding again aged 65 I can almost guarantee that you will find it very difficult to maintain a cadence of 90.

There's a lot of misunderstanding about cadence. People will tell you it is more efficient, but that depends on what you mean by efficient. Actually the reason most beginners use a cadence of around 60, as you do, is that to do so is the most efficient in terms of oxygen consumption for a given power output. So at a cadence of 60 you are placing less strain on your cardiovascular system than you would by adopting a cadence of 90 while maintaining a similar speed.

So, why do people advise a higher cadence? Because it is less tiring for the muscles, as well as being easier on the joints. Low cadence is easy on the heart but tough on the legs. High cadence, the reverse. So the reason that experienced cyclists and the very fit use a high cadence is because they can: they have power to burn, so to speak, so they can afford to put more stress on the CV system in order to ease the pressure on their muscles, and thus go faster, for longer. But beginners typically do not have the aerobic capacity to do this, if they try to use a cadence of 90 straight off the bat they feel like a demented hamster and tire themselves out.

Enjoy the ride. As you get fitter, your cadence is likely to increase naturally. To help it along a bit, try to pedal most of the time in a gear that is one lower than you think you could manage - this will challenge you enough to improve your fitness, and be kind to your knees, without exhausting you. And don't sweat the numbers. Some professionals use cadences lower than 90, people differ.