Malcolm gives his year of birth as 1806 and Janet 1803.
No they didn't actually say that. The UK Census
never gives a year of birth. It tells you what the enumerator thought the person said his or her age was on the day of the census. In 1841 adults' ages were supposed to be rounded down to the nearest 5 years, but I see that Janet's is recorded as 38 according to the transcription at
http://www.freecen.org.uk/cgi/search.pl. She should have been listed as 35, but no matter.
In the 1851 Janet's age is shown as 53 and her birthplace as Maybole, Ayrshire. So at least one of the censuses is wrong, as she cannot have been 38 in 1841 and 15 years older just under 10 years later! However you don't say that you have been unable to find Janet's birth and parentage, so I assume that you have that information.
You should of course look up the originals of both censuses on SP to check that the transcriptions are accurate.
I don't see Malcolm in the 1851 census, however. It may be that he was somewhere that hasn't yet been transcribed for FreeCEN, or perhaps his name was recorded with some even more creative spelling than McLarty or McClartie.
Interesting to note that there seem to be quite a few McLartys/McClarties listed in Argyll. Is it possible that Malcolm was actually born in Argyll and the enumerator got it wrong?