If it helps my glider was built in 1968.
It has never had any fatigue-related repairs to the fibreglass structure - all repairs have been due to impact damage of some kind. So far as I and my inspector can see, the fibrelass is as structurally sound as the day it was complete. Normally, the only structural maintenance on a glider is for metal parts (or where metal parts such as hinges have become delaminated from their attachment points because of repeated stressing).
UV damages coatings (paint or gel) but not the underlying structure so far as is known. There is the potential for damage if the coating is chipped allowing moisture in long term, but I think (though am not sure) that the main damage this causes is flaking off the surrounding coating.
For an aircraft which is not stored under cover gel coat is a no-no - a glider which has sat out for a year is a sorry sight. The right paint should be as good as on metal.
Skilled repairers can make repairs invisible - crashed gliders have been rebuilt from little more than shards.
The best comparison might be motorgliders like the Grob 109, some of which must be 30 years old. Many live outside, and seem to bear up well.
[edited to ask SkyHawk what there was on the course that put him off composite?]