Even gliders have airframe lives.
A Discus is initially 6,000 hrs, an inspection at that point raises it to 9,000. Further inspections at 10,000 and 11,000 hrs and then buy a new one at 12,000 hrs. That's a plastic airframe - mainly carbon.
Most Blaniks are now grounded (metal airframe) if over something like 3000 hours due fatigue problems in main spar. A mod exists to extend life - but costs more than the gliders are worth.