James, what do you suppose causes low gastric acid over time? why does this occur?
A big factor with age related drops in stomach acid production is a lack of methylation. Here is a link to an old post on the subject:
http://curezone.com/forums/fm.asp?i=1708234#i
There are other factors though such as the lack of stomach acid leads to decreased absorption of nutrients needed to produce stomach acid. A lack of acid can lead to heartburn as well. People tend to take antacids or acid blockers (proton pump inhibitors) when they have heartburn, which again inhibits the absorption of nutrients needed to produce stomach acid.