I'm not sure of the casualty rate for aircraft, but more crewmembers from Halifaxes survived (29%) than Lancaster crews (11%) - the Halifax had a bigger escape hatch in the nose, which most of the crew would have used. "The low rate for Lancasters was thought to be due to the fact that it was more difficult to move about in this narrow aircraft to the small forward compartment and to the poor rear escape hatch. It was also thought possible that the Lancaster was more liable to break up in the air and that the higher flight conditions might increase the difficulties of escape after the removal of the oxygen masks." (Operational Research in the RAF, (HMSO, 1963) p.66)
The average age became lower because many of the older men were pre-war regulars, many of whom were lost earlier in the war. Their replacements were mostly young men in their late teens, keen to fly and arguably more able to learn their trades quickly than those at the upper end of the age range.