60% young 30% mature 10% old, is how i go about my management tasks on a few areas i manage. you will get the hang of looking at the condition of the animal, look at the gate, look at the spine, look at the ribs, look at its behavoiur.
many years ago i ignored these basic ruels and left a reasonable buck running about on 3 legs, with the idea that he was good breeding stock! and yes the leg missing was a back one so copulation would have been awkward to say the least! i didnt realise it had 3 legs until observing its behaviour on a long term basis and looking at its condition / gate. needles to say it was culled and within a matter of days 2 newbies entered his range and made it there own!
good luck
Skipp.