I've had the same trouble with Red Cored Chantenay and Danvers Half
Long. I'm trying Healthmaster - that high vitamin A carrot - this year.

I too was surprised to find that the older carrots were sweeter. I
would have thought that "young and tender" would be preferable. But
perhaps that has to do with the purpose that I have for them. I try to
lay in 40-50 pounds for the winter. Because my storage "root cellars"
are old cold chests in the garage, I can't harvest until the weather
gets cold. So perhaps my younger bunch were yellow-orange and not very
sweet because they were started in July and harvested in November,
instead of being started in spring and harvested in the heat of summer
as yours may be.

Kathy