Here is an interesting study that says that vasodilation – the relaxing of blood vessels that is needed for erections, and which is improved by Viagra – declines with age; however, the good news is that exercise reverses this trend (at least, in rats). The researchers note that our old friend nitric oxide seems to be the agent that mediates these changes.

So, to counter loss of erection strength – exercise!

Here is a link to the article:

http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1665008&blobtype=pdf