Although not everyone reads the books and will consider that vaild information (they aren't canon, afterall), it's touched upon that basicly, the Founders can't die of old age, but are ofcourse not immortal. The sickness in the Great Link almost killed them, and we've seen atleast three Founders die onscreen (The Adversary, Apocolypse Rising and By Inferno's Light).
Interestingly enough, right now I'm reading the story by David R George III where some backstory is explained about the Founders, and how they used to be solids (as stated in one of the episodes of DS9, so that IS canon) and how the Progenitor, a massive changeling, made them all changelings. They were then considered newly formed, not infants, and couldn't procreate anymore. Even at the time of Odo's return, there were still newly formed Changelings in the Link, who hadn't seperated yet from the Link.