While not in the game UI, it is still not too hard to change. Go to the change appearance section of the ruler designer. Then click the Copy DNA button at the bottom. Then paste it into a text editor like Notepad. Find gene_baldness={ "male_pattern_baldness" XX "male_pattern_baldness" XX }. Replaces the numbers (in example shown as XX) with a number from 0-255. The first number is active gene that affects your character. The second number is your inactive gene that has a chance of being passed on instead of your active gene (kinda like dominant and recessive genes though the probability is not quite the same). Once the genes have been edited, copy everything (i.e. all the genes) and then click the Past DNA button inside the ruler designer. Note the gene will not have any affect while in the ruler designer, only once in game will you see the effect.
The number is the age at which the gene turns on for adult men with 0 being age 0 and 255 being age 100 (note this gene has no effect on women). Below are some values you can put in for some specific ages (edit: certain hair styles, at double their initial balding age, have a stage 2 level of balding):
Age 20: 51 (edit: age 25 is coded to be the minimum balding age, though this will still make for an earlier stage 2 level of balding)
Age 40: 102
Age 60: 153
Age 80: 204