The Age Gap - or, where are all the 60 & 70 year-olds?

In 1841 there were no Kerr folk on Harris in their 60s or 70s. None.
In the surrounding areas there were between 8% and 12% of such people in their populations which, if translated to the situation on Harris where there were 65 Kerrs, means that we would expect to see perhaps half-a-dozen people of that generation.
Instead, there is a gap between a few in their fifties and one lady, Chirsty of Taransay, who is said to be 80.
If they were a settled population going back a couple or more generations then we would surely expect at least one person in this age group to have been present? I think this is clear evidence that those in their 40s and 50s (born 1781-1801) amongst the population of 1841 were in fact the first generation to be born on Harris and that it was their parents who came to the island attracted by the call for craftsmen that went out when Macleod began his Improvements in the 1780s.
These parents would have been of Chirsty's generation (she herself may have been the mother of Roderick of Rha and others) and thus it is less surprising that they were no longer present by 1841. My own ancestors, Malcolm Kerr and Effie Shaw, parents of John and Angus, were of this generation and are the only ones whose children survived, or remained on Harris, beyond 1855 and the introduction of Statutory Registration. Hence my being able to identify them from their sons death certificates. If they had any daughters then, sadly, I would have to examine every female death of those born, say, 1780-1820 from 1855 onwards in the hope of discovering them beneath their married names. The same is true for the others of this 'original' generation.
A task too far!

If anyone can suggest another explanation for this unusual feature then I would be delighted to hear from you, but I think that mine is the simplest to fit the known facts, including the activities being undertaken on the island at the time?
"Captain Macleod also encouraged shoemakers, weavers, turners, wrights and masons to settle down at Rodel." (John Lanne Buchanan quoted in 'Travels to terra incognita', Martin Rackwitz 2007)


"he alfo encouraged a great many artificers; as fhoemakers, weavers, turners, and wrights, and mafons" (John Lanne Buchanan in his 'Travels in the Western Hebrides: from 1782 to 1790')


Note: There are two records from the 1851 census that show ages of 74 and 77 (and therefore people who would have been in their 60s in 1841) but the one who survives until the 1861 census resorts to a birth in the late 1780s so I think the 'gap' as described is real rather than merely an artifice of the records.