I would say at 70, if you able to get an erection enough for sex, you are ahead of the game or at least on par. Many men on these boards in their 50s, and some even their 40s, say they can only go once.  
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I am curious what your refractory time is, generally, since you mentioned it. If you are able to go twice in a RW date (or p4p one) in just a few hours or less, I would think you would be in the top 10-20% at your age, but this is just a guess as I am no expert.

There are so many variables. For me the girl makes all the difference. Some have it some don’t.

to two or more hours, you are probably not going to want to continue the hourly donation while you wait for Mr. Johnson to wake up from his nap.  You should only book enough time for one shot.  They go back daily if you want to.