We are a family of 5 (2 adults, 3 kids) we have stayed at POR twice now and found that on our last trip our family was getting too big for a single room. Oldest child was 11 at the time and he tried to sleep on the trundle bed one night and that was enough for him...he is very tall.
I would not suggest a trundle bed for a person over 10, it just would not be comfortable...remember a trundle bed mattress is just a crib mattress.