Oxalic acid content does increase with age (it is inthe stalks as well
as the leaves, although in the stocks it is in low quantites) A lot of
people do not like thae taste of oxalic acid and find the early season
stuff tastes better.

The only reason I use chalk in my rhubarb wine is that it helps reduce
the acid a bit for easier fermentation. I use about 4-5 lbs/ gallon
with a clove/ginger addition recipe.