There were lots and lots of young men flying in Lancasters, but the oldest crew members (not very many) were usually gunners (and in their 40's).
Many of the flight engineers were former Halton "Brats" chosen for their engineering knowledge, and likely to be above the average age. Likewise some of the wireless operators would have had prior experience in a ground wireless trade.
Would it not have been possible for the whole or part "greenhouse" on top to be made jettisonable ?
Still not easy to climb out that way (stand on the nav's table?) and then you'd need to avoid the inner propellers, the wing and the tailplane. Having climbed over the main spar with the aircraft on the ground, in a lit hangar and wearing a suit (me, not the aircraft!) I wouldn't want to do it wearing flying kit, with the aircraft pitching and rolling, at night. That said, most of the crew (pilot, engineer, navigator, bomb aimer and wireless operator) were forward of the spar, so they'd go for the nose hatch.