According to Andrew Roth at Club for Growth, the law mandating lifetime Secret Service protection for former Presidents was changed in 1997, limiting protection to 10 years after a President leaves office.
The first President to be subject to this new restriction? George W. Bush.
This seems stunningly stupid to me. The same Congress that refuses to reform its bridge-to-nowhere earmarks system finds protecting the lives of former Presidents to be prohibitively expensive.