Hi marvin,
you're absolutely right, your penis (actually your whole body) becomes less sensitive with age. This is just the weakening of your nerve endings which happens over time. You'll notice you don't even feel getting pinched on the arm as much as you did in your younger years.
However, you're in luck.
There are many supplements on the market that will increase your orgasm load... Most notibly is zma, tribulus, and tongat ali. All of which are available at your local health store. You can also look into some liquid ginseng or horny goat weed extract (liquid) to increase your libido a bit.
Also note that how sensitive you are has a lot to do with how you're feeling mentally.. If you focus, you will most definately be more sensitive and be able to reach orgasm.
Hope this helps,
j