What % of airbags don't deploy?

[Sprintey]

OK, not sure if this one has been done before, but...

Relative was recently T-Boned by a senior going through a stop sign. Car was a write off, as was, I suspect, the small car that did the hitting.

Side airbag did deploy in the rellies car, but the word is it deployed but there was no air in it, so the driver hit their head on the side glass and suffered some bruising. Little car doing the T-boning was not so lucky - airbag didn't deploy at all and result was broken sternum.

So that's two from two, and enough to make any engineer raise their eyebrows.

I'd estimate the ages of the cars as at 5 years, and maybe 7 years. Do airbag systems have a used by date? Might as well we all assume they won't deploy?