All bourbon tastes immature or perhaps it is more accurate to say not-fully-mature, until reaching a certain age. This age will vary depending on the the distilling-out proof, entry proof, aging conditions and other factors.
Recently on my trips to Bardstown I bought bottles of younger HH whiskey to scope the taste of its "young make". I bought Heaven Hill's white label which I think is 4 years old, the Heaven Hill BIB, and Rittenhouse BIB. Of these the first two showed the taste of young whiskey which is spicy and a little raw or gamy-like. This is a taste that the young bourbons of all the distillers have - I noticed it in the regular Ancient Age I bought out in Santa Rosa in July. The white label of HH offers the taste, as the regular Ancient Age does, in a pretty unvarnished form. The HH BIB swaths it in older oak tastes - I would guess most of what is in HH BIB is from the slower-maturing parts of the warehouse but the whiskey is older than that used in the HH white label.
My recent tastings of Evan Williams Black Label show more of this "distillery" taste than I remember, maybe because of the recent use of some whiskey under 7 years old.
I think this taste, which is an acquired taste to a degree, is the real 1800's taste of bourbon because most bourbon then wasn't aged past 4 years and much of it was probably 1-4 years old. Plus, a lot of it was made in pot stills or inefficient column beer stills. This is why I think cocktails became popular, because that kind of whiskey tastes good in cocktails. I find that using whiskey for a cocktail that is too neutral from long aging and too tannic from the barrel wood produces a mediocre cocktail.
It is not that I don't like the taste of older whiskey but the latter depends more on the wood for the character or the blending of wood tastes you get from batching. The gamy and spicy tastes get aged out in other words leaving wood and alcohol as the predominant tastes (and esters and other by-products of the congener conversion).
Four Roses Small Batch, using whiskeys which average 7 years of age, gets an especially good balance for me of youngish whiskey and excellent cask wood flavors. That kind of whiskey makes the best cocktail - where the base whiskey combines younger and older tastes. But FRSM is so good it drinks neat very well, too. And so does HH BIB I think but to drink the latter neat you really have really to understand and appreciate straight whiskey.
If I had some HH white label (I gave mine to Tim), I'd blend, say 25% of that with 75% of an older, fully matured whiskey such as EWSB 1996 (or half the latter and half EC 18 year old, and so on). One could play with the percentages. The young whiskey would enliven the older and the older would mollify and make more approachable the younger.
I am glad HH and the other distillers still sell young bourbon. It offers a taste which mixes well, which many people like neat or on rocks, and it comes at a good price. And for the historically minded, the taste of young whiskey probably represents what a lot of bourbon in the 1800's was like.