oh, didn't know. Then they
a) they never had the (best) equipment in Cup they did in lower series
b) they didn't face as good competition in lower series
c) they were faster when they were younger, underachieved in Cup
d) combo of a, b & c
That makes some to little sense, though. What constitutes good competition? I mean if we are to go by what those drivers he raced achieved then and now in cup, then I'd say Dale Jr. (since I don't nor have followed Biffle prior to cup and only follow him to rebuke arguments) had good competition. His main rival then was Matt Kenseth, who when he made it to cup was fast and still is fast, Junior himself when he got to cup was fast and still is fast. Thus that debunks the "younger thing" there are plenty of great drivers getting up in age and still don't have a championship due to the domination and driving in a JJ era. Also to touch on the talent one last bit, they had to do what current busch/nationwide/xfinity drivers do and that is race Cup regulars the good and the mediocre.
However, that isn't an excuse rather it just says JJ is better. I wouldn't say they didn't have the best equipment either, Junior had great equipment at DEI especially at plate tracks where their setup was the top one, but 2007 seemed to change this and multiple engine failures from the group left Junior out of the chase for the second time. It's ironic considering the big fallout between him fighting for ownership of the company which his sister would have helped him run (and probably would exist still and be a HMS supplied team most likely), and the fact that after the battle he jumped ship announcing he'd drive for HMS.
At HMS he had what could be argued the best equipment since JJ was winning championships every year up to that point and after until Stewart in 11. Junior had a bad 2009 and 2010 season but if it isn't due to any of the reasons you say then, why? Quite simply put, he just had bad seasons like any driver. Also more easily forgotten reasons some drivers never got a handle on the COT gen 5 cars and he was notoriously known to have this issue as was Gordon. He also had issues with crew chiefs (Eury but more notable Mcgrew), this didn't help him as a driver either and it becomes more obvious when we see how he did after the arrival of Latarte. Also when we see a switch to the Gen 6 car.
Even Biffle (though I don't like him) isn't a bad driver, it could be argued that team mates Edwards and former Kenseth are better but there were plenty of seasons were Biffle did good or great in the chase with this and 2011 being his awful years.
Point is there is far more to factor than just those simple portions and really those aren't even the taking points. Especially when Junior himself recently said he is still in his prime, but is cautious of falling off in a few years.