I had the same reaction you had, but thought the figure was opportunistic enough for me to use (I know, I'm totally biased), particularly because it came from a source that I would be capable of passing off as serious (University of Arkansas Division of Agriculture).

Apparently, hens mature at 6 months, so here they would be culled at 1/5th or 1/6th their adult age, which applied to humans being adults at 16-18 would make it, say, 3 years of age, so the age part is passibly accurate. (ps: I wonder whether hens are said to be adults at puberty, in which case the appropriate human age would be, what, 12 years?)

The average chicken adult body weight is at 1.5 kg, so killing them aged 6 weeks and weighing 2.5 kilos is 1.6x too much weight. (this is just a rough calculation, a quick one too, just to see if the 349 lbs statement is off or not)

Applied to humans, that would be 2-3 years of age, and 1.6 times the average human weight, and since I don't know that figure, I can't get the correct result, but I'm starting to think that the 349 lbs for 2 years can't be that far off.