I've never ran into this one personally but I was told about it by a reliable source! Feel free to add to this!
Now strictly speaking, Methuselah does not have a body but it can materialise! When it gets stronger, dark dust appears in the immediate area. This dust is almost weigthless and usually floats around in clouds one or two feet above the ground (and it vaguely smells like copper or rotten flesh)! D: This dust is really connected to Methuselah on a spirit level so technically you can vacuum it up and banish it (but that is pretty hard)! D:
Methuselah does not possess objects but rather feels attracted to atmospheres... A lingering atmosphere of despair or death will sustain it. It feeds off the lifespan of the living in near vicinity! Among the symptoms are fatigue, continuous hunger, apathy (which eventually beats out the hunger!), skin turning grey and accelerated aging. D: Classical places to find it are of course retirement homes, cemeteries, jails and hospitals!
Methuselah appears to be... repelled by something that appears energetic and lively but does not actually live (i.e. where there is no lifespan to take). The best example would be a flickering fire! That doesn't work on a big scale but if you go exploring an infected place, take a torch with you and you'll be safe for a while.
There is this story that Methuselah appeared on a battlefield where no side could turn the battle to their favour. It got so strong that on the third day that the soldiers all just put down their weapons and lay down to die. At the next dawn, what was left were piles of people who had died of old age.