Had a sliding door shatter when my dad ran into it years ago at my brother's apt. The glass probably shattered from heat stress or had a small knick in it and the sun stressed it and it shattered. How old was it?

It was 15 years old.

The workmen cleaned it up for me. Looking at $600+ to replace the tempered glass and muntins (sp?). The glass company said that sometimes glass spontaneously shatters right in their workrooms!

might be covered by homeowners insurance after the deductible. Tempered glass will shatter like that. Just like a car windshield.

Found a thread on the windows forum that you may be interested in...

Here is a link that might be useful: Sliding Glass Door - Shattered

Car windshields are not tempered glass, they are safety glass.
Two layers of glass with a plastic layer between them (originally developed and patented by Ford and sold under the ÂAutolite brand). It is designed to both resist breakage and contain occupants in a crash.
Side and rear windows are tempered glass, designed to break into 'crumbs' and not be a danger.

Tempered glass shatters when a scratch is deep enough to penetrate into the un-tempered core of the pane. It cannot be cut after tempering, and must be handled carefully. The edges are a very vulnerable spot. Even a small scratch there can shatter the pane.

Small scratches can propagate with thermal cycling through the temper layer to the core.

Tempered glass is both harder and more flexible than straight glass, but once the limits are exceeded the entire panel is destroyed almost instantly as the residual stresses from the tempering process are relieved.

There have also been issues with older double panes being overstressed as the argon fill gas leaked out faster than air could enter. The panes bow into the middle until they are overstressed and shatter.

Ambient temperature was 70°, doesn't account for thermal stress, and/or any type of smoke finish, factory tint, or (gasp!) aftermarket tint applied by the homeowner. All multiply thermal stress exponentially - tempered glass or not.