I wouldn't subject a dog to extreme weather conditions for long periods of time either!.... But a lab is built to withstand cold water for short periods of time and they do just fine.
As for a young, healthy, dog.... I am certain a lab would not get cold indoors in a room even if it felt cold to a person. I am assuming a person wouldn't keep a room at 35 or 40 degrees.
My labs do live inside with me... and I like it to be warm.... but they still have the ability to hunt in icy waters for short periods at a time and not show any ill effects.
They also love to roll and play in the snow and they always last longer than I do.
My shepherd seems to be more sensitive to the cold than my labs.
I would also like to say... I don't agree with keeping any dog in extreme or uncomfortable conditions.