Good questions raised Big5... I have made the mistake of branding the bulls according to my experience and have not given my reasons for doing so. Indeed they are not 'young' bulls, but the reason i judged them as immature is due to the fact that they would most likely not be shooters on a buffalo safari. On the close-up photo (thanks to Dugaboy1), it may appear that they have hard bosses, but they are actually just beginning to form a rough layer at the top. If you were to press your thumb on these bosses (where they meet together), you will find that the top crust will move down upon pressure, meaning that the bottom of the bosses is yet to fully form.
Further evidence is from the facial features. There is no typical wear and tear on the ridge of the nose and the 'tear-path' from the eyes. The facial hair is very intact which is not a rule, but very common among old bulls. Furthermore, you will note that the horns are still angling backwards (also not a rule but common of younger bulls) plus they have 'flick-back' tips which suggest little wearing down. On the close-up, the bull facing left is the older of the two and may in some places qualify as a last day bull, but it is not an appropriate bull to take down. Even though they are alone, it does not mean that they are old bachelor bulls. Buffaloes also enjoy young bachelorhood and then rejoin a herd when they are ready to fight for breeding rights.
Indeed, it is impossible to judge when a wild buffalo has 'peaked' or is in 'decline', but we all depend on the tell-tale signs. I have seen some really old bulls still hand around the herds and even sniff a cow or two and have seen very young bulls hand around with old dugga boys - there is no fixed rule to these habits. But in the case of the two candidates on the photo above, they would deserve a second and third glance maybe, but if the area has buffalo, then they are to be left alone for a future date
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In reference to the 'seam of the bosses coming together', that is another misconception that many hunters have. The distance between one boss and the other is very much a genetic quality and cannot be used entirely as an age indicator. It is the HARDENING of the bosses and COMPLETE formation of the bosses according to the genes of that particular bull that safely establish whether a bull is mature or not. They could be close together or up to six inches apart. A few photos here to illustrate various boss genetics - one meets at the seam, another is 2" apart and the other 3" apart - but they are all as old a bull as you could hope for.