What is the biological/evolutionary advantage to baldness?
Baldness would appear to be a trait on display. That being said, hair loss can be selected against the opposite condition (full head of hair), or selected by way of precise pattern deemed attractive. So, male pattern baldness evolution for mate selection purposes is probably a factor in play. But, either way, hair loss appears to have both its advantages and disadvantages as far as both fitness and mate selection is concerned. 

Similarly, in terms of fitness, the advantage/disadvantage you state is the immediate positive advantage of hair as a protectant.  The immediate advantages of hair diminish as one grows older, making age apparent. Old age is disadvantageous in itself (older can mean less fit). Hence, the immediate advantage for a protection against the elements decreases and becomes less relevant to survival as one grows older. Please see the references below.

https://en.wikipedia.org/wiki/Mate_choice

https://en.wikipedia.org/wiki/Evolution_of_ageing