The rule is that anyone baptised and anointed at the point of time of 1914 (or is it 1919?) constitute the 1st generation.
So taking your example, someone born in 1839 could be baptised and anointed by 1859. If they lived until they were 80 (1919) then they are the 1st generation.
Another example. Fred Franz was baptised and anointed by 1914 but lived until 1977 or something. This means he was also part of the 1st generation because he was anointed at the key point in time - 1914.
Both these examples show that the 1st generation stretches from the 1830's to the 1970's.... ish. 140 years for one generation.
BUT... "This Generation" is actually TWO "generations" which overlap, so...
If someone was baptised and anointed by the 1970's, and overlapped Fred Franz, maybe being 20 years old at time (i.e. born in the mid 1950's), this person is ALSO part of "This Generation" because he is in the 2nd generation group - the overlapping group.
If this person lives until they are 80 years old (let's say until 2030), then "This Generation" that Jesus said will never pass away actually stretches from the 1830's to the 2030's - 200 years!
WT have obviously got this prophecy solved. After all, 7 millions JW's all say it makes sense.
Splash