If you are old at 50, then a lot of us are definitely geriatrick. Couple of points when starting to ride that are not mentioned. Hills and Saddles. Both will cause you pain initially. Both will stop being a problem after a few hundred miles practice. On the saddle front, try to wear in your existing saddle for around 2 months. It probably is not the saddle at fault initially, so buying a new saddle may not cure the problem.
Hills. Take them steady, and do not shy away from them either. If you are unfortunate enough to find a riding partner that is younger or fitter than you, then let him go at the hills at his speed. If it is slower than you want to go, then you will be lucky, but do them at your pace.
The only other aid to cycling at your age that can be recomended is a Heart Monitor. If won't tell you if you are over doing it, or if the heart attack is coming, but if you use it as an aid to tell you that you are reaching your limits, it will stop you running out of energy too soon on your ride. As an example, at 50 you should have a maximum heart rate of 172, (if you go by the accepted rule of 222 less your age) It will take a lot of energy to reach this initially so unless you are fit, do not try and get to it. I generraly get my HR up to 130, rest to 120. up to 140 rest to 125, up to 150 rest to 130. Then try to hold 145 for 5 minutes,and rest to 130. If at the end of this I am feeling fine, then I know I am fit enough to reach the 165 max for me at any time of the ride, so I can push myself. If I am tired, or breathing too heavy, then I know I will be taking an easier ride, for a while, until the heart rate will settle out again. You don't need an expensive monitor for this. The cheapest in the range will do, but you may want to go up a grade to get extra functions, but they are not necessary. One thing a heart rate will tell you though, is how much you have improved over a years riding.

The other point to remember at our age, is that We can't stay with the youngsters, they have too much energy. What we can do though is outlast them on endurance. Or at least that is what I keep telling them when they laugh at me for being the last one up the hills, until 30 miles in when they want to go home.