I think Allen is in a fantastic shape at this point in time. Dude is 47 years old, which is well beyond the point many(most?) energetic rock/metal singers lose range/start sounding tired/throw pronounciation overboard/etc.
Recent performances I stumbled accross:
https://www.youtube.com/watch?v=o_TIR_Lq3rkTransiberian Orchestra. Here he sings clean in lower pitch, later moving on to full on rock belting.
https://www.youtube.com/watch?v=lT5dOPQYmOoBon Jovi cover. Here he is in an acoustic setting (he sings the higher parts in the chorus).
As for the discussion of what affects the vocals of singers, offcourse good technique is an extremely important part in maintaining the voice, but it isn't the only factor. I do think, no matter the technique, extensive touring and high density/long duration of singing during concerts can lead to vocal fatique. Just like the overuse of basically any part of your body. Powering through sickness and vocal issues for a long time also obviously isn't going to do you any favours in the long term (which a lot of touring singers tend to do). And then there is offcourse ageing. Ageing affects people differently to a high degree. Control over muscles plays a big role in singing (and basically any type of controlled movement of a bodily part). These capabilities simply decrease as people age, and at different rates for any person (just like, say, hand eye coordination slowly declines over the years). It is not just the vocal cords that (can) take a hit over time.