Of the 3 ... nutrition, old age, & fenced area ... fenced areas that create captive deer that can reach fully expressed age characteristics through manipulation of diet, stress drivers & levels, selective breeding, selective harvest, etc. The repeatability and consistency of high percentile growth capabilities becomes the norm and not the exception.
In true free range deer, you will need to have the right nutrition in place to support growth. But without a fence, bucks stand little chance to reach an age to take advantage of nutrition.
I don't think many people understand how much nutrition is manipulated in fenced/captive operations. There is no comparison in what can be given in a pen and what mother nature can produce even under the best of circumstances.
That is the same reason why a chicken can be grown from egg to a 5 lbs table ready bird in 6-7 weeks.