The slow down: Doof notices that Perry has started to slow down during one of their regular engagements
Doof didn't bother naming his inator's anymore, not since he realized that they had less and less to do with a real or imagined slight from his childhood, and not because he didn't enjoy naming them, but because he had run out of names and ideas. He had been rehashing old ideas together without Perry even noticing. He sighed as he awaited Perry. A cup of tea in hand as he tried to predict how Perry would come in this time. All the more surprise as he heard the door creak open. Perry was opening the door instead of kicking it down? That was unusual. Perry was breathing heavily as well. That was even more unusual. Even if he had come up the stairs Doof knew that their almost daily interactions kept Perry fit. So why was he breathing as though it hurt?
"Here Perry why don't you sit down and catch your breath?"
Perry gave him a look as though asking what was up "No traps I promise. Just sit and catch your breath. When you are done then you can blow up my latest inator."
Looking at Perry Doof noticed that there were some small patches of fur missing from Perry's coat, and there, there was a grey hair.
'Just how old is he?' Doof thought as he watched Perry get up, and Doof halfheartedly pretended to stop him. Thinking back he remembered that Perry had been assigned as Doof's nemesis just a mere 10 years prior. As far as he knew from his brief service as a good guy at OWCA, Perry had been an agent for far longer than that. He cursed Perry's name as he watched him fly away, but his mind was already on something else. Going deep into his lair he found an old encyclopedia set and found the entry platypus. An hour of frustrated reading later and he closed the book for the fifth time. There was no mention of how long a platypus could live. Although he did learn that male platypi, platypeople, platypus whatever had venomous spurs. He wondered about that, wouldn't Perry have used such an advantage if he had it? Just another thing to add to his growing list of concerning things he didn't know about Perry. Well there was only one thing for it, watch for his nemesis to make sure the little fellow was all right, and if he was getting old, Doof would make sure that with a little blackmail the monotreme would get the retirement he deserved. Doof owed him that much.
End
Wikipedia did not have any reference as to how long a platypus lives. Did learn some other neat facts though. If any one knows please feel free to let me know. And I figure that the 10 year thing is from flashbacks. We know Doof didn't get Perry as a nemesis until at least after he got the Doofensmirtz building, which would be after his divorce, and assuming again from flashbacks that Vanessa was a toddler when Doof divorced we can assume that Perry has been Doof's nemesis for at least 10 years. Vanessa is 16 right? so that sounds about right.