A lot of us choose to wear black trousers for all occasion regardless of the time of day. But take this photo of Yours Truly, taken at a Malay wedding reception in Bangsar recently. Note the surrounding colours - the building in the background, its doors, the grass ( not really seen in the pictures) and other bits.
There is not an ounce of black in them and in the natural surrounds. Black (especially for trousers) is limited to shadows during the day. Black should be used for evenings, dinners, formal events, funerals, for mournings and for waiters.
I suppose another reason for someone with a little more white hair as well as the lack of it, is that one should compensate for the slight lack of vigour (compared to a twenty year old) by adding some flair in their clothing. Wearing black when you're twenty is very acceptable as you have that extra energy that seems to radiate from youth.