Probably varies some as to when the bosses "harden". You can have a big, wide, heavy, fully mature, herd bull that is hard bossed but not particularly old at maybe 7-10 years and definitely not considered a dagga boy. And it probably varies quite a bit as to when they start to become loners or spend a lot of time with other bulls and separated from the main herd living mostly as dagga boys. Old bulls can have craggy, ridged or chipped bosses or even bosses where the ridges have been smoothed but that is different from soft bosses. I don't believe a truly old bull (probably older than 10 or 12) will ever have soft bosses. If you see hair between bosses or don't see ridges or craggy/chipped surfaces or ridges that have been smoothed it will not be an old bull. Really old bulls may actually start losing mass of the bosses and give the appearance of receding away from each other leaving a widening gap between.