It's no doubt on his mind Bucky... but, he is also a high-performing athlete who, like all the rest, possesses a HUGE inner-drive and competitive edge. There have only been 6 QB's in the history of the NFL to throw for more than 20 TD's in a season at the age of 38. And only 2 QB's to do it at 39.
There is nothing about Kurt's performance to suggest he's slowing down or wouldn't put up healthy numbers again next season. Additionally, Kurt has always said his primary drive is to win a Championship for this franchise. Clearly, there are a TON of factors beyond his own situation that will play into our ability to contend again next season... so I'm sure Kurt is talking to Whiz and Graves about their plans for this offseason and what they'll do about Q, Dansby, Left Tackle, Secondary, Berry, Okeafor, etc...
Who knows... in the end he just might decide he's had enough. However, that would be extremely rare for a guy who is still one of the very best at his position...